run-tests: fix regular expression for path test

The previous regexp would match the empty string. This fixes
a mistake added in 964212780daf.

Differential Revision: https://phab.mercurial-scm.org/D1843

tests/run-tests.py

@@ -2450,7 +2450,7 b' class TestRunner(object):'
             # If it looks like our in-repo Rust binary, use the source root.
             # This is a bit hacky. But rhg is still not supported outside the
             # source directory. So until it is, do the simple thing.
-            elif re.search(b'|/rust/target/[^/]+/hg', normbin):
+            elif re.search(b'/rust/target/[^/]+/hg', normbin):
                 self._pythondir = os.path.dirname(self._testdir)
             # Fall back to the legacy behavior.
             else: