bpo-37064: Skip test_tools.test_pathfix if installed

[vstinner]

If Python is installed, skip test_tools.test_pathfix test because
Tools/scripts/pathfix.py script is not installed.

https://bugs.python.org/issue37064

[vstinner]

I tested manually my fix:

./configure --with-pydebug --prefix /opt/py39 && make clean && make && make install
cd /
/opt/py39/bin/python3.9 -m test test_tools -v

The test is now skipped as expected:

vstinner@apu$ /opt/py39/bin/python3.9 -m test test_tools -v
== CPython 3.9.0a0 (heads/pathfix_installed:8c7504c8b4, Sep 5 2019, 17:42:14) [GCC 9.1.1 20190503 (Red Hat 9.1.1-1)]
== Linux-5.2.8-200.fc30.x86_64-x86_64-with-glibc2.29 little-endian
== cwd: /tmp/test_python_12197
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 load avg: 0.93 [1/1] test_tools
test.test_tools.test_fixcid (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'
test.test_tools.test_gprof2html (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'
test.test_tools.test_i18n (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'
test.test_tools.test_lll (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'
test.test_tools.test_md5sum (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'
test.test_tools.test_pathfix (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'
test.test_tools.test_pdeps (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'
test.test_tools.test_pindent (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'
test.test_tools.test_reindent (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'
test.test_tools.test_sundry (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'
test.test_tools.test_unparse (unittest.loader.ModuleSkipped) ... skipped 'scripts directory could not be found'

----------------------------------------------------------------------

Ran 11 tests in 0.002s

OK (skipped=11)

== Tests result: SUCCESS ==

1 test OK.

Total duration: 127 ms
Tests result: SUCCESS