tests: unify exec framework

[casperdcl]

Merge any test using Bash.compgen/zsh_spec_array/fish_candidates/tcsh_candidates into a parameteri[sz]ed test.

e.g. replace test_fish_file_completion(change_dir, test_parser) with:

exec_candidates = {'zsh': zsh_spec_array, 'fish': fish_candidates, 'tcsh': tcsh_candidates, ...}
exec_shell = pytest.mark.parametrize('shell', list(exec_candidates))

@exec_shell
test_file_completion(shell, change_dir, test_parser):
    (change_dir / "test_file.txt").touch()
    completion = shtab.complete(test_parser, shell)
    assert ... in exec_candidates[shell](completion, ...)
    ...

also misc tidy, e.g.:

• rework Bash.compgen -> bash_candidates (test: unify exec framework #209)
• rework zsh_spec_array -> zsh_candidates

/CC @ThomasWaldmann