hghave: allow adding customized features at runtime Before this patch, there is no way to add customized features to `hghave` without changing `hghave` and `hghave.py` themselves. This decreases reusability of `run-tests.py` framework for third party tools, because they may want to examine custom features at runtime (e.g. existence of some external tools). To allow adding customized features at runtime, this patch makes `hghave` import `hghaveaddon` module, only when `hghaveaddon.py` file can be found in directories below: - `TESTDIR` for invocation via `run-tests.py` - `.` for invocation via command line The path to the directory where `hghaveaddon.py` should be placed is added to `sys.path` only while importing `hghaveaddon`, because: - `.` may not be added to `PYTHONPATH` - adding additional path to `sys.path` may change behavior of subsequent `import` for other features `hghave` is terminated with exit code '2' at failure of `import hghaveaddon`, because exit code '2' terminates `run-tests.py` immediately. This is a one of preparations for issue4677.

import-checker.py: exit with code 0 if no error is detected Before this patch, `import-checker.py` exits with non-0 code, if no error is detected. This is unusual as Unix command. This change may be a one of preparations for issue4677, because this can avoid extra explanation about unusual exit code of `import-checker.py` for third party tool developers.

run-tests.py: add TESTDIR to PATH if it differs from RUNTESTDIR Before this patch, `RUNTESTDIR` is added to `PATH`, but `TESTDIR` isn't. This doesn't cause any problems, if `run-tests.py` runs in `tests` directory of Mercurial source tree. In this case, `RUNTESTDIR` should be equal to `TESTDIR`. On the other hand, if `run-tests.py` runs in `tests` of third party tools, commands in that directory should be executed with explicit `$TESTDIR/` prefix in `*.t` test scripts. This isn't suitable for the policy "drop explicit $TESTDIR from executables" of Mercurial itself (see 4d2b9b304ad0). BTW, 4d2b9b304ad0 describes that "$TESTDIR is added to the path" even though `TESTDIR` isn't added to `PATH` exactly speaking, because `TESTDIR` and `RUNTESTDIR` weren't yet distinguished from each other at that time. This is a one of preparations for issue4677.