tests: make sure test-run-tests.t actually runs run-tests.py under Python 3

I'm fairly certain it hasn't been until now. Mercifully, there doesn't
appear to be any ninja breakage.

Differential Revision: https://phab.mercurial-scm.org/D3349

tests/test-run-tests.t

@@ -6,8 +6,7 b' Avoid interference from actual test env:'
 
 Smoke test with install
 ============
-
-  $ run-tests.py $HGTEST_RUN_TESTS_PURE -l
+  $ $PYTHON $TESTDIR/run-tests.py $HGTEST_RUN_TESTS_PURE -l
   
   # Ran 0 tests, 0 skipped, 0 failed.
 
@@ -15,14 +14,14 b' Define a helper to avoid the install ste'
 =============
   $ rt()
   > {
-  >     run-tests.py --with-hg=`which hg` "$@"
+  >     $PYTHON $TESTDIR/run-tests.py --with-hg=`which hg` "$@"
   > }
 
 error paths
 
 #if symlink
   $ ln -s `which true` hg
-  $ run-tests.py --with-hg=./hg
+  $ $PYTHON $TESTDIR/run-tests.py --with-hg=./hg
   warning: --with-hg should specify an hg script
   
   # Ran 0 tests, 0 skipped, 0 failed.
@@ -31,7 +30,7 b' error paths'
 
 #if execbit
   $ touch hg
-  $ run-tests.py --with-hg=./hg
+  $ $PYTHON $TESTDIR/run-tests.py --with-hg=./hg
   usage: run-tests.py [options] [tests]
   run-tests.py: error: --with-hg must specify an executable hg script
   [2]