Changeset 37beb92 in mod_gnutls for test/runtests

Test suite: Do not explicitly set the mutex type to "default"

The setting can cause trouble when the mutex type "default" is file
based and its definition includes a path that the build process cannot
write to. This problem caused the Debian build to fail on hurd-i386,
where "default" resolved to "file:/var/run/apache2/" according to the
build log.

According to the HTTPD documentation a run-time file directory
relative to ServerRoot? is used absent an explicit setting, and the
ServerRoot? defined in test/base_apache.conf must be writable for the
test suite anyway.