test-run-tests: do not rebuild hg in the test

d600bda4 and fc0f3ed0 added code to call `$PYTHON run-tests.py ...`. That
will rebuild hg, is slow and could have other crashes from setup.py, like:

  Unable to find a working hg binary to extract the version from the
  repository tags

Therefore use `run-tests.py -l` instead.

Differential Revision: https://phab.mercurial-scm.org/D1595

tests/test-run-tests.t

@@ -1280,7 +1280,7 b' support for automatically discovering te'
   $ cp -R tmp tmpp
   $ cp tmp/test-uno.t test-solo.t
 
-  $ $PYTHON $TESTDIR/run-tests.py tmp/ test-solo.t tmpp
+  $ rt tmp/ test-solo.t tmpp
   .....
   # Ran 5 tests, 0 skipped, 0 failed.
   $ rm -rf tmp tmpp
@@ -1303,7 +1303,7 b' support for running run-tests.py from an'
   > EOF
 
   $ cd ..
-  $ $PYTHON $TESTDIR/run-tests.py tmp/test-*.t
+  $ rt tmp/test-*.t
   
   --- $TESTTMP/anothertests/tmp/test-folder-fail.t
   +++ $TESTTMP/anothertests/tmp/test-folder-fail.t.err