check.mak: Valgrind fails to find tests written in tests/check/ directly (rather than a subdir) - because valgrind ge...

Original commit message from CVS:
* check.mak:
Valgrind fails to find tests written in tests/check/ directly (rather
than a subdir) - because valgrind gets run with a filename that
doesn't contain a relative path, it goes searching /usr/bin instead.
Run with ./.... to make things work either way.

* gtk-doc-plugins.mak:
Add $(top_builddir)/src as a place to look for plugins
when building too, since that's where gst-template keeps things