Improve testing for v3.3; misc cleanup

[khaeru]

This PR:

• Adds tests of the setting/usage of "message model dir" per Setting the message model dir does not work  #415. The issue is not closed, as it awaits info.
• Cleans up some mess in the test suite introduced by old PRs.
  In particular, test_timeseries.py was improved in Test using in-memory databases and other clean-ups #270: duplicate tests were collected into a smaller number of simpler, parametrized tests that covered more behaviour, and these were collected in a class. These improvements were then squashed/reversed by Extend API to support dynamic time slices #264; see e66f3f8#diff-153145079fe9f6531ebb1e54aa22d353d2c89112fea960469a548ed0768a11ec. The PR corrects the error while preserving and improving additions over the past year. (Time: about 4 hours.)
• Improves performance in the test suite.
• Improves test coverage, towards Increase test coverage to 100% #386.
• Adjusts the config and platform CLI commands to use click properly; adds config show to show the config file location.
• Adds genno and message-ix-models to ixmp show-versions.
• Remove documentation of rixmp, parallel and linked to Update docs for v3.3.0; remove rmessageix message_ix#473.

How to review

Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

Merging #416 (1bfdbdd) into main (0188de0) will increase coverage by 1.1%.
The diff coverage is 97.0%.

@@           Coverage Diff           @@
##            main    #416     +/-   ##
=======================================
+ Coverage   96.7%   97.9%   +1.1%     
=======================================
  Files         37      39      +2     
  Lines       4247    4238      -9     
=======================================
+ Hits        4111    4150     +39     
+ Misses       136      88     -48     

Impacted Files	Coverage Δ
ixmp/__init__.py	100.0% <ø> (+9.0%)	⬆️
ixmp/backend/__init__.py	100.0% <ø> (ø)
ixmp/model/dantzig.py	100.0% <ø> (ø)
ixmp/tests/core/test_scenario.py	100.0% <ø> (ø)
ixmp/tests/test_config.py	100.0% <ø> (+4.4%)	⬆️
ixmp/utils.py	96.0% <ø> (+0.9%)	⬆️
ixmp/testing/resource.py	83.0% <83.0%> (ø)
ixmp/backend/jdbc.py	95.0% <90.9%> (-1.1%)	⬇️
ixmp/testing/__init__.py	95.8% <95.8%> (ø)
ixmp/testing/data.py	97.2% <97.2%> (ø)
... and 24 more

[khaeru]

See #417.