tests/test-hghave.t

extensions: load help from hgext.__index__ as a fallback this time

Prior to 843418dc0b1b, `hgext.__index__` was consulted first if present, which
caused the longer help from the extension modules to be ignored, even when
available.  But that change causes a bunch of test failures when the pyoxidized
binary bundles *.pyc in the binary, saying the there's no help topic for
`hg help $disabled_extension` and suggesting the use of `--keyword`, rather than
showing a summary and indicating that it is disabled.  Current failures were in
test-check-help.t, test-extension.t, test-help.t, and test-qrecord.t.

Ideally, we would read the various *.pyc files from memory and slurp in the
docstring, but I know that they used to not be readable as resources, and I
can't figure out how to make it work now.  So maybe 3.9 and/or the current
PyOxidizer doesn't support it yet.  I got closer in py2exe with
`importlib.resources.open_binary("hgext", "rebase.pyc")`, but `open_binary()` on
*.pyc fails in pyoxidizer.[1]  Either way, the *.pyc can't be passed to
`ast.parse()` as `extensions._disabledcmdtable()` is doing, so I'm setting that
aside for now.

[1] https://github.com/indygreg/PyOxidizer/issues/649

  $ . "$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" $TESTDIR/run-tests.py --with-hg=$HGTEST_REAL_HG -j 1 \
  >    $HGTEST_RUN_TESTS_PURE test-hghaveaddon.t \
  > )
  installed Mercurial in * seconds  (glob) (?)
  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]