check: allow config tests to pass if necessary

When MPM DSOs are enabled, Apache::Test's invocation of `httpd -V` runs
a full configuration test. To pass, ensure that *all* instances of the
configured prefix path in the test configuration are pointing to a real
directory in check/.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/trunk-test-integration@1778026 13f79535-47bb-0310-9956-ffa450edef68