Multitest generates tests into single folder, even if tests have same names

[alsemenov]

Please, make sure you are familiar with https://github.com/dart-lang/sdk/blob/master/tools/testing/dart/multitest.dart

The problem is dicovered with co19 test suite.
Let's imagine we have two tests in the suite:
col9\Language\Expressions\Logical_Boolean_Expressions\syntax_t10.dart

class A {}

main() {
  A || true; /// 01: static type warning, dynamic type error
}

and col9\Language\Expressions\Shift\syntax_t10.dart

class A {}

main() {
  1 >> A; /// 01: static type warning, runtime error
}

The tests have the same file name, but they reside in different folders.
The tests have different expected results: dynamic type error and runtime error

So, when test suite is executed, there is wrong test result:

co19/Language/Expressions/Logical_Boolean_Expressions/syntax_t10/01: RuntimeError

Executing the test manually shows that test result is wrong.
The investigation shows, that test files are generated to the same folder out/platform/generated_tests/co19. So there is only one set of generated tests syntax_t10_none.dart syntax_t10_01.dart with the content, that corresponds to col9\Language\Expressions\Shift\syntax_t10.dart test. If I make the test case keys unique,
then three files are generated syntax_t10_none.dart syntax_t10_key1.dart syntax_t10_key2.dart, which is not good either. (Although this fixes the wrong test result problem).

This problem blocks fixing dart-lang/co19#69 because we have modified a lot of tests and are not able to execute the test suite before committing.

[whesse]

Fixed by https://codereview.chromium.org/2270413002/ .
Let me know if this doesn't work for you.