Enforces having unique test ids in a single namespace

[sobolevn]

I found lots of duplicate test names. It might hide important errors, be hard to reproduce or rerun.

So, let's enforce having unique tests ids.
Note: that some tests have a quite complex name like testFollowImportsNormalPackage2-only_when_cache, where only_when_cache is not really its name, but a running condition.
So, I had to ignore this part and only use name part.

Refs #11662

[sobolevn]

Awesome, we even got a newly discovered test failure!

___________________________ testExportModule_import2 ___________________________
[gw1] linux -- Python 3.10.0 /home/runner/work/mypy/mypy/.tox/py/bin/python
data: /home/runner/work/mypy/mypy/test-data/unit/stubgen.test:486:
/home/runner/work/mypy/mypy/mypy/test/teststubgen.py:562: in run_case
    self.run_case_inner(testcase)
/home/runner/work/mypy/mypy/mypy/test/teststubgen.py:595: in run_case_inner
    assert_string_arrays_equal(testcase.output, a,
E   AssertionError: Invalid output (/home/runner/work/mypy/mypy/test-data/unit/stubgen.test, line 486)
----------------------------- Captured stderr call -----------------------------
Expected:
  import re as re                               (diff)
                                                (diff)
  x: int                                        (diff)
Actual:
  x: int                                        (diff)
  y: int                                        (diff)

Alignment of first line difference:
  E: import re as re
  A: x: int
     ^
=========================== short test summary info ============================
FAILED mypy/test/teststubgen.py::StubgenPythonSuite::stubgen.test::testExportModule_import2
====== 1 failed, 9654 passed, 370 skipped, 7 xfailed in 687.98s (0:11:27) ======
ERROR: InvocationError for command /home/runner/work/mypy/mypy/.tox/py/bin/python -m pytest -n 2 (exited with code 1)

[sobolevn]

@hauntsaninja thanks for solving the conflicts! 👍