[PATCH v2] user32/tests: Use winetest contexts to simplify test_string().

[Marvin]

Hi,

While running your changed tests, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?

Full results can be found at:
https://testbot.winehq.org/JobDetails.pl?Key=91807

Your paranoid android.


=== w1064 (64 bit report) ===

user32:
clipboard.c:761: Test failed: 7: gle 5
clipboard.c:766: Test failed: 7.0: got 0000 instead of 0011
clipboard.c:807: Test failed: 7: gle 1418
clipboard.c:817: Test failed: 7: count 3
clipboard.c:820: Test failed: 7: gle 1418
clipboard.c:854: Test failed: 7: format 0011 got data 0000000000B0AB70
clipboard.c:855: Test failed: 7.0: formats 00000000 have been rendered
clipboard.c:860: Test failed: 7.0: formats 00000000 have been rendered
clipboard.c:854: Test failed: 7: format 0002 got data 000000003E05075E
clipboard.c:855: Test failed: 7.1: formats 00000000 have been rendered
clipboard.c:860: Test failed: 7.1: formats 00000000 have been rendered
clipboard.c:855: Test failed: 7.2: formats 00000000 have been rendered
clipboard.c:860: Test failed: 7.2: formats 00000000 have been rendered