Problem:    Various inconsistencies in test files.
Solution:   Add modelines where they were missing.  Use Check commands instead
            of silently skipping over tests.  Adjust indents and comments.
            (Ken Takata, closes #6695)

Problem:    assert_fails() checks the last error message.
Solution:   Check the first error, it is more relevant.  Fix all the tests
            that rely on the old behavior.

Problem:    Crash when using garbagecollect() in between rand().
Solution:   Redesign the rand() and srand() implementation. (Yasuhiro
            Matsumoto, closes #5587, closes #5588)

Problem:    No test with wrong argument for rand().
Solution:   Add a test case.

Problem:    rand() does not use the best algorithm.
Solution:   use xoshiro128** instead of xorshift. (Kaito Udagawa,
            closes #5279)

Problem:    Using time() for srand() is not very random.
Solution:   use /dev/urandom if available

Problem:    Random number generator in Vim script is slow.
Solution:   Add rand() and srand(). (Yasuhiro Matsumoto, closes #1277)