GH-113528: Split up pathlib tests for invalid basenames. (#113776)

Split test cases for invalid names into dedicated test methods. This will
make it easier to refactor tests for invalid name handling in ABCs later.

No change of coverage, just a change of test suite organisation.

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -215,10 +215,13 @@ class DummyPurePathTest(unittest.TestCase):
         self.assertNotEqual(P(), {})
         self.assertNotEqual(P(), int)
 
-    def test_match_common(self):
+    def test_match_empty(self):
         P = self.cls
         self.assertRaises(ValueError, P('a').match, '')
         self.assertRaises(ValueError, P('a').match, '.')
+
+    def test_match_common(self):
+        P = self.cls
         # Simple relative pattern.
         self.assertTrue(P('b.py').match('b.py'))
         self.assertTrue(P('a/b.py').match('b.py'))
@@ -398,14 +401,17 @@ class DummyPurePathTest(unittest.TestCase):
         self.assertEqual(P('/').anchor, sep)
         self.assertEqual(P('/a/b').anchor, sep)
 
-    def test_name_common(self):
+    def test_name_empty(self):
         P = self.cls
         self.assertEqual(P('').name, '')
         self.assertEqual(P('.').name, '')
+        self.assertEqual(P('/a/b/.').name, 'b')
+
+    def test_name_common(self):
+        P = self.cls
         self.assertEqual(P('/').name, '')
         self.assertEqual(P('a/b').name, 'b')
         self.assertEqual(P('/a/b').name, 'b')
-        self.assertEqual(P('/a/b/.').name, 'b')
         self.assertEqual(P('a/b.py').name, 'b.py')
         self.assertEqual(P('/a/b.py').name, 'b.py')
 
@@ -448,10 +454,13 @@ class DummyPurePathTest(unittest.TestCase):
         self.assertEqual(P('a/Some name. Ending with a dot.').suffixes, [])
         self.assertEqual(P('/a/Some name. Ending with a dot.').suffixes, [])
 
-    def test_stem_common(self):
+    def test_stem_empty(self):
         P = self.cls
         self.assertEqual(P('').stem, '')
         self.assertEqual(P('.').stem, '')
+
+    def test_stem_common(self):
+        P = self.cls
         self.assertEqual(P('..').stem, '..')
         self.assertEqual(P('/').stem, '')
         self.assertEqual(P('a/b').stem, 'b')
@@ -470,11 +479,17 @@ class DummyPurePathTest(unittest.TestCase):
         self.assertEqual(P('/a/b.py').with_name('d.xml'), P('/a/d.xml'))
         self.assertEqual(P('a/Dot ending.').with_name('d.xml'), P('a/d.xml'))
         self.assertEqual(P('/a/Dot ending.').with_name('d.xml'), P('/a/d.xml'))
+
+    def test_with_name_empty(self):
+        P = self.cls
         self.assertRaises(ValueError, P('').with_name, 'd.xml')
         self.assertRaises(ValueError, P('.').with_name, 'd.xml')
         self.assertRaises(ValueError, P('/').with_name, 'd.xml')
         self.assertRaises(ValueError, P('a/b').with_name, '')
         self.assertRaises(ValueError, P('a/b').with_name, '.')
+
+    def test_with_name_seps(self):
+        P = self.cls
         self.assertRaises(ValueError, P('a/b').with_name, '/c')
         self.assertRaises(ValueError, P('a/b').with_name, 'c/')
         self.assertRaises(ValueError, P('a/b').with_name, 'c/d')
@@ -488,11 +503,17 @@ class DummyPurePathTest(unittest.TestCase):
         self.assertEqual(P('/a/b.tar.gz').with_stem('d'), P('/a/d.gz'))
         self.assertEqual(P('a/Dot ending.').with_stem('d'), P('a/d'))
         self.assertEqual(P('/a/Dot ending.').with_stem('d'), P('/a/d'))
+
+    def test_with_stem_empty(self):
+        P = self.cls
         self.assertRaises(ValueError, P('').with_stem, 'd')
         self.assertRaises(ValueError, P('.').with_stem, 'd')
         self.assertRaises(ValueError, P('/').with_stem, 'd')
         self.assertRaises(ValueError, P('a/b').with_stem, '')
         self.assertRaises(ValueError, P('a/b').with_stem, '.')
+
+    def test_with_stem_seps(self):
+        P = self.cls
         self.assertRaises(ValueError, P('a/b').with_stem, '/c')
         self.assertRaises(ValueError, P('a/b').with_stem, 'c/')
         self.assertRaises(ValueError, P('a/b').with_stem, 'c/d')
@@ -506,10 +527,16 @@ class DummyPurePathTest(unittest.TestCase):
         # Stripping suffix.
         self.assertEqual(P('a/b.py').with_suffix(''), P('a/b'))
         self.assertEqual(P('/a/b').with_suffix(''), P('/a/b'))
+
+    def test_with_suffix_empty(self):
+        P = self.cls
         # Path doesn't have a "filename" component.
         self.assertRaises(ValueError, P('').with_suffix, '.gz')
         self.assertRaises(ValueError, P('.').with_suffix, '.gz')
         self.assertRaises(ValueError, P('/').with_suffix, '.gz')
+
+    def test_with_suffix_seps(self):
+        P = self.cls
         # Invalid suffix.
         self.assertRaises(ValueError, P('a/b').with_suffix, 'gz')
         self.assertRaises(ValueError, P('a/b').with_suffix, '/')