Move tests/* into */tests/

The tests in the $top_srcdir/tests directory and its subdirectories are crufty. Since forever we have been putting tests in $top_srcdir/{gio,gobject,glib}/tests, which makes more sense.

For each test in $top_srcdir/tests, we should:

• Check whether it’s a duplicate of a more modern test, and drop any entirely-duplicated old test coverage (checking that this doesn’t reduce our code coverage, either overall or for the specific code being tested)
• Port it to use GTest (g_test_add(), g_test_run(), etc.) so that it can be run by a modern test framework and its results be successfully interpreted by machine (using TAP)
• Move it to $top_srcdir/{gio,gobject,glib}/tests

This can be done piecewise.

To do:

• asyncqueue-test.c: !2347 (merged)
• atomic-test.c: !1123 (merged)
• bit-test.c: !2379 (merged)
• child-test.c: !2390 (merged)
• completion-test.c: !2392 (merged)
• cxx-test.cpp: !2391 (merged)
• datetime.c: !2393 (merged)
• dirname-test.c: !2397 (merged)
• env-test.c: !2399 (merged)
• file-test.c: !2404 (merged)
• gio-test.c: !2539 (merged)
• gobject/accumulator.c: !2550 (merged)
• gobject/defaultiface.c: !2550 (merged)
• gobject/deftype.c: !2550 (merged)
• gobject/dynamictype.c: !2550 (merged)
• gobject/gvalue-test.c: !2428 (merged)
• gobject/override.c: !2550 (merged)
• gobject/paramspec-test.c: !2431 (merged)
• gobject/performance-threaded.c: !2695 (closed), !2735 (merged)
• gobject/performance.c: !2695 (closed), !2735 (merged)
• gobject/references.c: !2550 (merged)
• gobject/signals.c: !2550 (merged)
• gobject/singleton.c: !2550 (merged)
• gobject/testgobject.c: !2696 (merged)
• gobject/testmodule.c: !2550 (merged)
• gobject/timeloop-closure.c: !2697 (merged)
• iochannel-test.c: !2502 (merged)
• mainloop-test.c: !2645 (merged)
• mapping-test.c: !2607 (merged)
• memchunks.c: !2579 (merged)
• module-test.c: !2660 (merged)
• onceinit.c: !2601 (merged)
• qsort-test.c: !2370 (merged)
• refcount/*: !2553 (merged)
• refcount/closures.c: !696 (merged)
• relation-test.c: !2559 (merged)
• run-assert-msg-test.sh
• run-collate-tests.sh: !2434 (merged)
• slice-color.c: !2564 (merged)
• slice-concurrent.c: !2564 (merged)
• slice-test.c: !2579 (merged)
• slice-threadinit.c: !2580 (merged)
• sources.c: !2376 (merged)
• spawn-test-win32-gui.c: !2575 (merged) / !2599 (merged)
• spawn-test.c: !2575 (merged)
• testgdate.c: !2433 (merged)
• testgdateparser.c: !2526 (merged)
• testglib.c: !2424 (merged)
• thread-test.c: !2639 (merged)
• threadpool-test.c: !2634 (merged)
• timeloop-basic.c: !2609 (merged)
• timeloop.c: !2609 (merged)
• type-test.c: !2527 (merged)
• unicode-caseconv.c: !2432 (merged)
• unicode-collate.c: !2434 (merged)
• unicode-encoding.c: !2581 (merged)
• unicode-normalize.c: !2586 (merged)

Note that currently none of the tests listed in tests/Makefile.am are touched by tests/meson.build.