This feature is primarily intended for use in plugin modules' unit tests.

Consider the following situation: gst-plugins-good is built against an
installed GStreamer core. An older version of gst-plugins-good is also
installed in that prefix, along with random other plugin modules. Now,
when doing 'make check' in the just-built gst-plugins-good tree, we
want to only load plugins from GStreamer core, gst-plugins-base, and
gst-plugins-good, but not random other modules (we don't want any unit
tests to fail just because some module in gst-plugins-bad has a broken
plugin_init, for example). Also, we want to only load gst-plugins-good
modules from the locally-built source tree, but not any of the older
gst-plugins-good modules installed. This is usually assured by loading
the ones in the source tree first (by adding that path first to the
right environment variables), but it gets tricky when plugins are
moved, removed, merged, or renamed, or the plugin filename changes.

Note that 'make check' should really work right without doing
'make install' or uninstalling the old gst-plugins-good package (or
any other gst-plugins-foo package) first.

Enter GST_PLUGIN_LOADING_WHITELIST. This environment variable may
contain source-package@path-prefix pairs separated by the platform
search path separator (G_SEARCHPATH_SEPARATOR_S). The source package
and path prefix are separated by the '@' character. The path prefix is
entirely optional, as is the '@' separator if no path is given.

It is also possible to filter based on plugin names instead of the name
of the source-package by specifying one or more plugin names separated
by commas before the optional path prefix.

In short, the following match patterns are possible:

   plugin1,plugin2@pathprefix or
   plugin1,plugin2@* or just
   plugin1,plugin2 or
   source-package@pathprefix or
   source-package@* or just
   source-package

So for our gst-plugins-good unit test example above, we  would set the
environment variable on *nix to something like this (will likely be a
relative path in practice):
gstreamer:gst-plugins-base:gst-plugins-good@/path/to/src/gst-plugins-good

Fixes #619815 and #619717.