test: fix self._testdir to use the right mercurial library during testing
Currently if you run tests from some other directory other than the
`../tests/`, you will get a warning stating
```
warning: Testing with unexpected mercurial lib: mercurial
(expected /tmp/hgtests.xxxxxx/install/lib/python/mercurial)
```
This is because the current directory being added to the 'PATH', if the
`self._testdir != runtestdir`, owing to this line
```
if self._testdir != runtestdir:
path = [self._testdir] + path
```
Also say you ran the tests from the hg base directory,
because directory is being added in the PATH (see the above snippet, at
that stage the `self._testdir` has the value as `cwd`, owing to a faulty
initialization). And since the current directory already has the 'hg',
that is used in place of the hg that is installed for the testing purposes
in `/tmp/hgtests.xxxxxx/...`.
Differential Revision: https://phab.mercurial-scm.org/D5199
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.