git-annex test from standalone speedup

git-annex test from standalone speedup

Avoid git-annex test being very slow when run from within the standalone
linux tarball or OSX app.

It may not really be necessary to add to PATH the directory where the
git-annex binary resides, but it can't hurt. Most places where the test
suite or git-annex run git-annex, they use programPath, so won't need
a modified PATH. But I'm not sure if that's always the case.

Sponsored-by: Dartmouth College's Datalad project