run-tests: fix conditional when tests are run outside of `tests`

The logic to find `hghave` was using `__file__` but… non absolute path in
`__file__` gets wrong as soon as we change directory (thanks python… you are
being helpful). The rest of the `run-tests.py` code already deal with this
fine. We simply reuse the `RUNTESTDIR` variable to fix it.

tests/run-tests.py

@@ -1626,7 +1626,7 b' class TTest(Test):'
             return self._have.get(allreqs)
 
         # TODO do something smarter when all other uses of hghave are gone.
-        runtestdir = os.path.abspath(os.path.dirname(_bytespath(__file__)))
+        runtestdir = osenvironb[b'RUNTESTDIR']
         tdir = runtestdir.replace(b'\\', b'/')
         proc = Popen4(
             b'%s -c "%s/hghave %s"' % (self._shell, tdir, allreqs),