The bzr tests are the only .t tests calling hghave (aside from the tests
testing hghave itself). This pattern is a one-off and prevents
desired refactors to how requirements checking is integrated into tests.
Use the convention used everywhere else.
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.
