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

@@ -1,44 +1,44 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; \
-  > $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.
 
 (invocation via command line)
 
   $ unset TESTDIR
   $ hghave custom
 
 (terminate with exit code 2 at failure of importing hghaveaddon.py)
 
   $ rm hghaveaddon.*
   $ cat > hghaveaddon.py <<EOF
   > importing this file should cause syntax error
   > EOF
 
   $ hghave custom
   failed to import hghaveaddon.py from '.': invalid syntax (hghaveaddon.py, line 1)
   [2]