Fix re-building test '127 custom target directory install' with VS backend

Running the build step of test '127 custom target directory install'
again, using the VS backend, causes 'docgen.py' to try to create the
target directory again (which fails with a FileNotFound exception).

I'm guessing that perhaps this is a shortcoming of the VS backend that
it doesn't correctly give this target a dependency on the directory.

I'm not sure that this test is actually valid meson: the reference
manual says custom_target(output:) should be a list of files, and not a
directory, as is this case here.

test cases/common/127 custom target directory install/docgen.py

@@ -5,7 +5,10 @@ import sys
 
 out = sys.argv[1]
 
-os.mkdir(out)
+try:
+    os.mkdir(out)
+except FileExistsError:
+    pass
 
 for name in ('a', 'b', 'c'):
     with open(os.path.join(out, name + '.html'), 'w') as f: