pythongh-109615: Fix support test_copy_python_src_ignore()

Fix the test when the test is run on an installed Python: get
abs_srcdir of sysconfig, and skip the test if the Python source code
cannot be found.

Author: vstinner

Lib/test/pythoninfo.py

@@ -520,6 +520,7 @@ def collect_sysconfig(info_add):
         'SHELL',
         'SOABI',
         'abs_builddir',
+        'abs_srcdir',
         'prefix',
         'srcdir',
     ):

Lib/test/test_support.py

@@ -802,8 +802,13 @@ def test_set_memlimit(self):
             support.real_max_memuse = old_real_max_memuse
 
     def test_copy_python_src_ignore(self):
-        src_dir = sysconfig.get_config_var('srcdir')
+        src_dir = sysconfig.get_config_var('abs_srcdir')
+        if not src_dir:
+            src_dir = sysconfig.get_config_var('srcdir')
         src_dir = os.path.abspath(src_dir)
+        if not os.path.exists(src_dir):
+            self.skipTest(f"cannot access Python source code directory:"
+                          f" {src_dir!r}")
 
         ignored = {'.git', '__pycache__'}