Add g_assert_cmpstrv() test utility

See #2015 (closed)

Marked this as a draft to see if this is the kind of output you expect from this macro. An easy way to look at the output is to change glib/tests/testing.c:test_assertions() and then to run meson test -C build --suite glib testing

Some things for discussion:

• I explicitly added a NULL-comparison, since it's a trap I fell into multiple times
• I didn't serialize the full string array, for fear of low-readable output if the string arrays that are being compared, are large.

Fixes: #2015 (closed)