test-module-imports: try and detect virtualenv breakage (issue4129)

virtualenvs (among other things) break the stdlib module detection in
the import checker, and I don't see a good way to work around that for
now.

tests/test-module-imports.t

@@ -2,6 +2,13 b' This code uses the ast module, which was'
 this test on anything earlier.
   $ python -c 'import sys ; assert sys.version_info >= (2, 6)' || exit 80
 
+Virtualenv has a habit of leaving BaseHTTPServer and other modules in
+a place where the import checker is confused about their nature as
+part of the stdlib. Skip the test if BaseHTTPServer's path isn't a
+subpath of sys.prefix.
+
+  $ python -c 'import sys, BaseHTTPServer; assert BaseHTTPServer.__file__.startswith(sys.prefix)' || exit 80
+
   $ import_checker="$TESTDIR"/../contrib/import-checker.py
 Run the doctests from the import checker, and make sure
 it's working correctly.