Test using in-memory databases and other clean-ups

[khaeru]

This PR improves the test suite in a variety of ways:

• The former 'testdb', a prepared HSQLDB, is removed. (There was no clear indication of what it contained, or any way to recreate it. This made it unnecessarily difficult to read tests.)
  • ixmp.testing.populate_test_platform() is added, which fills any Platform with equivalent content.
  • The test_mp fixture now refers to a temporary, in-memory database.
  • Some tests (only where needed) use test_mp and populate_test_platform() to generate the equivalent of the old testdb. Other tests, where possible, use a completely empty test_mp.
  • R tests are simplified because the R 'testthat' tests no longer need a path to an on-disk database.
• Test are moved inside the ixmp/ directory tree. This common practice means they can be packaged with the code and run by the user.
• Tests of ixmp.core are split into three files, one for each of the Platform, TimeSeries, and Scenario classes.
  • Tests for time-series data, geodata, and regions are grouped with the appropriate class.
• In test_access.py, the old-style tmpdir pytest fixture is replaced with the current tmp_path.
• Only a few minor changes to actual code:
  • TimeSeries.get_geodata() returns the 'year' and 'meta' columns as int, to be consistent with other code.
  • Scenario.check_out() checks if the Scenario has a solution, but TimeSeries.check_out() does not, because this is meaningless.
• Test coverage is increased from 95% to 97%.

How to review

Confirm that CI checks pass.

PR checklist

• Tests added.
• Documentation added.
• Release notes updated.

[codecov]

Codecov Report

Merging #270 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #270   +/-   ##
=======================================
  Coverage   96.96%   96.96%           
=======================================
  Files          38       38           
  Lines        3530     3530           
=======================================
  Hits         3423     3423           
  Misses        107      107           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7530ce3...7530ce3. Read the comment docs.

[danielhuppmann]

For the record, the file to create the testdb existed at least until v0.1.3 - tests/testdb_setup.py. But using an in-memory db is of course smarter.

Apart from that, I don't think that I can provide any useful review comments to that PR...