test: fix self._testdir to use the right mercurial library during testing

Currently if you run tests from some other directory other than the
`../tests/`, you will get a warning stating

```
warning: Testing with unexpected mercurial lib: mercurial
         (expected /tmp/hgtests.xxxxxx/install/lib/python/mercurial)
```

This is because the current directory being added to the 'PATH', if the
`self._testdir != runtestdir`, owing to this line

```
if self._testdir != runtestdir:
    path = [self._testdir] + path
```

Also say you ran the tests from the hg base directory,
because directory is being added in the PATH (see the above snippet, at
that stage the `self._testdir` has the value as `cwd`, owing to a faulty
initialization). And since the current directory already has the 'hg',
that is used in place of the hg that is installed for the testing purposes
in `/tmp/hgtests.xxxxxx/...`.

Differential Revision: https://phab.mercurial-scm.org/D5199

tests/run-tests.py

@@ -2543,17 +2543,18 b' class TestRunner(object):'
             os.umask(oldmask)
 
     def _run(self, testdescs):
+        testdir = getcwdb()
         self._testdir = osenvironb[b'TESTDIR'] = getcwdb()
         # assume all tests in same folder for now
         if testdescs:
             pathname = os.path.dirname(testdescs[0]['path'])
             if pathname:
-                osenvironb[b'TESTDIR'] = os.path.join(osenvironb[b'TESTDIR'],
-                                                      pathname)
+                testdir = os.path.join(testdir, pathname)
+        self._testdir = osenvironb[b'TESTDIR'] = testdir
         if self.options.outputdir:
             self._outputdir = canonpath(_bytespath(self.options.outputdir))
         else:
-            self._outputdir = self._testdir
+            self._outputdir = getcwdb()
             if testdescs and pathname:
                 self._outputdir = os.path.join(self._outputdir, pathname)
         previoustimes = {}
@@ -2901,7 +2902,7 b' class TestRunner(object):'
                 testcls = cls
                 break
 
-        refpath = os.path.join(self._testdir, path)
+        refpath = os.path.join(getcwdb(), path)
         tmpdir = os.path.join(self._hgtmp, b'child%d' % count)
 
         # extra keyword parameters. 'case' is used by .t tests