Build docs after running tests, and remove redundant DIST_SUBDIRS

Build docs after running tests, and remove redundant DIST_SUBDIRS

If DIST_SUBDIRS isn't set, it defaults to SUBDIRS, so it's just noise.

Running tests before building documentation is an easy way to speed up the
hack/make check/fix cycle, by not wasting time rebuilding the
documentation (which is often slow) until all the tests compile and pass.

https://bugs.freedesktop.org/show_bug.cgi?id=34405
Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>