fix sanitizer check failure via rate limit

[Guest0x0]

Recently the sanitizer check CI keeps failing randomly. Address sanitizer slows down the program, which makes the test program fail to finish all ready tasks before the next round of timeout expiration, eventually result in very high latency and incorrect test result. The solution here is to add a max concurrent test restriction to tests in moonbitlang/async. Currently the value is set to 20.

Another failing test is walk_test.mbt in @fs. With the recent introduction of README.mbt.md for the @fs package, a lot of tests in the README will be running at the same time at the beginning, again causing high latency. Since there is only one time-sensitive test in @fs, the solution here is to add a delay before that time-sensitive test, so that it won't race with the other tests without timeout.

[coveralls]

Pull Request Test Coverage Report for Build 688

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.543%

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Totals
Change from base Build 682:	0.0%
Covered Lines:	1164
Relevant Lines:	1482

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💛 - Coveralls