Make extensions in ext run their tests from the extension's own directory.

Make extensions in ext run their tests from the extension's own directory.

Inspired by, and in parts borrows from, Schwern's branch on github, but takes a
slightly different approach in places.

Not quite perfect yet - ext/File-Glob still runs from t, at least one FIXME
needs fixing, and the changes to dual-life modules' tests need to be filtered
back upstream, and possibly modified to suit their respective authors.

But it works.