tests: invoke run-tests.py in test-hghave using $PYTHON (issue5697)

Some platforms (notably pkgsrc on NetBSD) only provide versioned
Python interpreters (eg `python2.7` exists, but `python` does not),
which exposes this error. We want to be running run-tests.py with the
python given in $PYTHON, since that might be `pypy` or `python3`.

tests/test-hghave.t

@@ -22,7 +22,7 b' Testing hghave extensibility for third p'
   > EOF
   $ ( \
   > testrepohgenv; \
-  > $TESTDIR/run-tests.py $HGTEST_RUN_TESTS_PURE test-hghaveaddon.t \
+  > $PYTHON $TESTDIR/run-tests.py $HGTEST_RUN_TESTS_PURE test-hghaveaddon.t \
   > )
   .
   # Ran 1 tests, 0 skipped, 0 failed.