Move some tests from cmdline.test (#5966 )

[sloboegen]

Relates #5966.

Below is the info what happened with the concrete test from cmdline.test:

1. testErrorContextConfig => check-flags.test (testShowErrorContextFunction) [duplicate]
2. testNoConfigFile => check-flags.test (testNoConfigFile) [move]
3. testPerFileConfigSection => check-flags.test (testPerFileUntypedDefs) [move]
4. testIgnoreErrorsConfig => check-flags.test (testPerFileIgnoreErrors) [move]
5. testConfigFollowImportsNormal => check-flags.test (testFollowImportsNormal) [move + modified]
6. testConfigFollowImportsSilent => check-flags (testFollowImportsSilent) [move + modified]
7. testConfigFollowImportsSkip => check-flags (testFollowImportsSkip) [move + modified]
8. testConfigFollowImportsError => check-flags.test (testFollowImportsError) [move + modified]
9. testConfigFollowImportsSelective => check-flags.test (testFollowImportsSelective) [move]
10. testConfigSilentMissingImportsOff => check-flags.test (testSilentMissingImportsOff) [move]
11. testConfigSilentMissingImportsOn => check-flags.test (testSilentMissingImportsOn) [move]
12. testDisallowAnyGenericsBuiltinCollectionsPre39 => check-flags.test (testDisallowAnyGenericsBuiltinTuplePre39, testDisallowAnyGenericsBuiltinListPre39, testDisallowAnyGenericsBuiltinSetPre39, testDisallowAnyGenericsBuiltinDictPre39) [split]
13. testDisallowAnyGenericsTypingCollections => check-flags.test (testDisallowAnyGenericsTupleWithNoTypeParamsGeneric, testDisallowAnyGenericsPlainList, testDisallowAnyGenericsPlainDict, testDisallowAnyGenericsPlainSet) [split]
14. testDisallowUntypedDefsAndGeneric => check-flags.test (testDisallowUntypedDefsAndGeneric) [move]
15. testParseError => parse-errors.test (testMissingBracket) [move]
16. testParseErrorAnnots => check-fastparse.test (testFasterParseTooManyArgumentsAnnotation) [duplicate]
17. testNotesOnlyResultInExitSuccess => check-flags.test (testNotesOnlyResultInExitSuccess) [move]

Let's compare test execution time. I've run pytest -n 4 mypy/test/testcmdline.py 3 times on my machine and calculated the average time.

• Before: 130 tests, 1m 02s
• After: 115 tests, 0m 55s

Also, if it's possible to use fixture FrozenSet in check-flags.test, we'd be able to totally split items 12 and 13 from the above list.

And testMissingBracket is skipped by pytest in the parse-errors.test-file, but, probably, this file is the best variant for it (not sure about it).

[hauntsaninja]

Wow, this is great! Thank you for doing this, and thanks for your notes that made reviewing this PR much easier