tools/test.dart test script is too slow

[whesse]

The dart port of the test script takes more than 4 minutes to reach 99%, compared to 2 minutes for test.py. We should eliminate this difference. I'm measuring time to 99% due to some tests that timeout at the end.

When listing the tests only, we see that most of the time is spent generating multitests:
test.py --list: 3 seconds
test.dart --list: 2 minutes 30 seconds
test.dart --list (with multitests disabled): 16 seconds.

Disabling multitests makes the test.dart script take only 2 minutes 18 seconds for a full run, so the major performance problem can be solved by fixing this one issue.

[whesse]

Processing the multitests and making the output tests takes only 2 seconds, so it is creating the directories and writing the files that takes the extra 2 minutes 15 seconds.

Just creating the files takes 1 minute more time than doing nothing.
So opening, writing, and closing is taking 1:15 more.

Changing the creation of the multitests to use synchronous file access speeds this up
tremendously. Listing now takes 25 seconds, and the entire test script now takes 2:20, versus 2:00 for test.py. The issue can be closed, though there may be other minor improvements possible.

The fix is now out for review, and will be committed.

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Added Fixed label.