selftest: Add TESTENV_DIR "env" variable

selftest: Add TESTENV_DIR "env" variable

We store the testenv directory path for the 'ctx' hashmap, but not for
the testenv-vars hashmap (and that can be really annoying sometimes).
Add it into the second hashmap that selftest actually keeps track of.
Currently it's only stored in the hashmap, not actually exported as an
environment variable (but we could easily do that if a test-case need
this info).

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>