replace bundled tempdir with pytest tmpdir fixture

[orbeckst]

Many tests still use tests/tempdir.py. Remove and replace the use with the pytest tmpdir/tmpfile fixture.

[orbeckst]

Actually, tmpdir is kind of deprecated, use tmp_path

[cadeduckworth]

Working list: for removing setup & teardown

• test_Gsolv
• test_dihedral
• test_ensemble
• test_equilibration_script
• test_fep_script
• test_solv_shell

Working list: for changing tmpdir to tmp_path

• test_analysis
• test_analysis_alchemlyb
• test_solvation