tests: use test harness `hg` in test-hghave.t

This prevents the sub test harness from rebuilding Mercurial.
It shaves ~30s from the test wall time on my machine.

Differential Revision: https://phab.mercurial-scm.org/D6908

tests/test-hghave.t

@@ -1,46 +1,46 b''
   $ . "$TESTDIR/helpers-testrepo.sh"
 
 Testing that hghave does not crash when checking features
 
   $ hghave --test-features 2>/dev/null
 
 Testing hghave extensibility for third party tools
 
   $ cat > hghaveaddon.py <<EOF
   > import hghave
   > @hghave.check("custom", "custom hghave feature")
   > def has_custom():
   >     return True
   > EOF
 
 (invocation via run-tests.py)
 
   $ cat > test-hghaveaddon.t <<EOF
   > #require custom
   >   $ echo foo
   >   foo
   > EOF
   $ ( \
   > testrepohgenv; \
-  > "$PYTHON" $TESTDIR/run-tests.py -j 1 \
+  > "$PYTHON" $TESTDIR/run-tests.py --with-hg=`which hg` -j 1 \
   >    $HGTEST_RUN_TESTS_PURE test-hghaveaddon.t \
   > )
   running 1 tests using 1 parallel processes 
   .
   # Ran 1 tests, 0 skipped, 0 failed.
 
 (invocation via command line)
 
   $ unset TESTDIR
   $ hghave custom
 
 (terminate with exit code 2 at failure of importing hghaveaddon.py)
 
   $ rm hghaveaddon.*
   $ cat > hghaveaddon.py <<NO_CHECK_EOF
   > importing this file should cause syntax error
   > NO_CHECK_EOF
 
   $ hghave custom
   failed to import hghaveaddon.py from '.': invalid syntax (hghaveaddon.py, line 1)
   [2]