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.

== Local
Ready.
Un and Deux
Eins und Zwei
One and Two
Nope
Eins und Zwei
Hello, John Smith
Ready.
Uno und Due

== Remote
Ready.
REQ: foo?one=Vo&two=Efvy
  -> Vo!boe!Efvy
Un and Deux
REQ: bar?b=Fjot&a=[xfj
  -> Fjot!voe![xfj
Eins und Zwei
REQ: batch?cmds=foo:one=Pof,two=Uxp;bar:b=Fjot,a=[xfj
  -> Pof!boe!Uxp;Fjot!voe![xfj
REQ: greet?name=Kpio!Tnjui
  -> Ifmmp-!Kpio!Tnjui
REQ: batch?cmds=bar:b=Vop,a=Evf
  -> Vop!voe!Evf
One and Two
Nope
Eins und Zwei
Hello, John Smith
Ready.
Uno und Due