Reduce `mismatch` test run time

[AlexGuteniev]

Before this change the mismatch and lex compare test consumed more time than all other tests in vector algorithms test combined.

Done the following:

• Reused lgk mismatch result in lgk lex compare
• Reduced the number of random mismatches

[StephanTLavavej]

Thanks! On my local machine, this results in a test speedup of 1.61x, very nice! :heart_eyes_cat:

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for noticing and fixing how this test was wasting time! :stopwatch: :hourglass_flowing_sand: :heart_eyes_cat: