The Visual Studio builds actually copies the freshly built GIO
modules to the appropriate location upon completion of build, and
generates the giomodule.cache, so we don't really need the
GIO_EXTRA_MODULES envvar here, plus the test sources define it
according to the autotools build layout.

Also, update the TLS tests to not include GNUTLS items when we are
not building against the GNUTLS backend.