tests: fix basic.OutDirectoryIsExecutable

[perillo]

Commit 4b4cb9c (Merge pull request #426 from perillo/fix-argv-parsing) added a regression test, but the test failed due to ptrace errors on Linux.
The "Operation not permitted" error was caused by kcov trying to trace the "/bin/usr/python".

The solution is to execute an executable having user permission. Restore the short-test.py executable, and use it instead of python. short-test.py always exits with exit status 0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.72%. Comparing base (692d41c) to head (402b735).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #428      +/-   ##
==========================================
- Coverage   65.81%   65.72%   -0.10%     
==========================================
  Files          58       58              
  Lines        4520     4508      -12     
  Branches     4177     4165      -12     
==========================================
- Hits         2975     2963      -12     
  Misses       1545     1545              

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[perillo]

For the reason of the ptrace error with the old test, see
https://bugs.launchpad.net/ubuntu/+source/linux-signed/+bug/1860822

In Linux, process tracing is tied with the process dumping flag. The dumpable flag is cleared at execve(2) time when a setuid or setgid application is executed. This flag persists to child processes created by fork(2) and will only be reset when a process calls execve(2) again.

> stat -L /usr/bin/python3
Access: (0755/-rwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)

stat tests/python/short-test.py 
Access: (0750/-rwxr-x---)  Uid: ( 1000/  manlio)   Gid: ( 1000/  manlio)

The kcov test suite runs correctly on my system, without using sudo and with /proc/sys/kernel/yama/ptrace_scope set to 1 (the default value).

[SimonKagstrom]

OK, thanks for the PR and the well-made analysis! Merged!