[2.7] bpo-30283: regrtest: backport test_slow_interrupted() and test_coverage() ...

[2.7] bpo-30283: regrtest: backport test_slow_interrupted() and test_coverage() (#1541)

* bpo-30283: regrtest: backport test_coverage()

* Add --coverage option, the option was already described in the doc
* When coverage is used, regrtest now pass all options to runtest()
  and calls also accumulate_result() (as done when coverage is not
  used).
* bpo-25260: Fix coverage on Windows: remove the list of ignored
  directories.

* bpo-30283: regrtest: backport test_slow_interrupted()

* Fix regrtest to report interrupted tests as omitted rather than
  failed.
* bpo-25260: Fix coverage on Windows: remove the list of ignored
  directories.

* bpo-30283: Fix test_regrtest on Visual Studio 2008

Skip Tools\buildbot\test.bat and PCbuild\rt.bat if Python was not
compiled in PCbuild (but compiled in PC\VS9.0\ for example).