Many tests didn't change back from subdirectories at the end of the tests ...
and they don't have to. The missing 'cd ..' could always be added when another
test case is added to the test file.
This change do that tests (99.5%) consistently end up in $TESTDIR where they
started, thus making it simpler to extend them or move them around.
test-gpg has to be run in a mercurial working directory as it uses
that to verify that it hasn't modified the trustdb.gpg file. Skip the
test if it is running in an exploded tarball.
Tests really should not modify files in the Mercurial working dir
where they ran from! I suspect that trustdb.gpg is now old enough that
GPG thinks it should update it automatically. Suppress that feature
with --no-auto-check-trustdb.
