run-tests: ensure the script exits when it fails to change directories

Caught by PyCharm as an unused variable assignment.  This regressed in
c496e8c14b9e.

Differential Revision: https://phab.mercurial-scm.org/D7750

tests/run-tests.py

@@ -1839,7 +1839,7 b' class TTest(Test):'
                 cmd = rawcmd.split()
                 toggletrace(rawcmd)
                 if len(cmd) == 2 and cmd[0] == b'cd':
-                    l = b'  $ cd %s || exit 1\n' % cmd[1]
+                    rawcmd = b'cd %s || exit 1\n' % cmd[1]
                 script.append(rawcmd)
             elif l.startswith(b'  > '):  # continuations
                 after.setdefault(prepos, []).append(l)