tests: don't look for plugins in -base installdir for tests

tests: don't look for plugins in -base installdir for tests

-base plugins will always be found in the build directory, and
core plugins will be found either also via the build directory
(if both core and -base are a subproject) or by getting the
pluginsdir via pkg-config if core is installed.

The GST_PLUGIN_LOADING_WHITELIST env var will make sure we only
pick up plugins from core/base and base plugins only from the
builddir.

There is no reason to look for -base plugins in the install dir.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/582>