run-tests: stop ignoring venv-installed packages

This will allow test dependencies to be installed within a venv, instead of
tampering with sys/user sites.  One thing to note here is that the `VIRTUAL_ENV`
path takes precedence over system-site, unlike when run directly with an
activated `venv`.  For example, `sys.path` as viewed from a feature test in
`hghave.py`, when running `hghave vcr` directly with an activated `venv`:

    sys.path: [
        '/home/jenkins/hg-committed/tests',
        '/usr/lib/python38.zip',
        '/usr/lib/python3.8',
        '/usr/lib/python3.8/lib-dynload',
        '/home/jenkins/test_venv/lib/python3.8/site-packages'
    ]

vs `sys.path` from the same feature test, when run by `run-tests.py` with this
change:

    sys.path: [
        '/home/jenkins/hg-committed/tests',
        '/home/jenkins/hg-committed',
        '/home/jenkins/hg-committed/tests',
        '/home/jenkins/test_venv/lib/python3.8/site-packages',
        '/usr/lib/python38.zip',
        '/usr/lib/python3.8',
        '/usr/lib/python3.8/lib-dynload',
        '/home/jenkins/.local/lib/python3.8/site-packages',
        '/usr/local/lib/python3.8/dist-packages',
        '/usr/lib/python3/dist-packages'
    ]

tests/run-tests.py

@@ -3252,6 +3252,18 b' class TestRunner:'
         # adds an extension to HGRC. Also include run-test.py directory to
         # import modules like heredoctest.
         pypath = [self._pythondir, self._testdir, runtestdir]
+
+        # Setting PYTHONPATH with an activated venv causes the modules installed
+        # in it to be ignored.  Therefore, include the related paths in sys.path
+        # in PYTHONPATH.
+        virtual_env = osenvironb.get(b"VIRTUAL_ENV")
+        if virtual_env:
+            virtual_env = os.path.join(virtual_env, b'')
+            for p in sys.path:
+                p = _sys2bytes(p)
+                if p.startswith(virtual_env):
+                    pypath.append(p)
+
         # We have to augment PYTHONPATH, rather than simply replacing
         # it, in case external libraries are only available via current
         # PYTHONPATH.  (In particular, the Subversion bindings on OS X