Support focusing tests through fit keyword

[cegonse]

Tests may be focused defining tests through the fit keyword, as in:

fit("only this test will be executed", []() {
    expect(true).toBe(true);
});

it("will not be executed", []() {
    expect(true).toBe(true);
});

• JUnit test report must indicate all tests in the suite except the focused test have been skipped.
• Test output must show the SKIPPED badge for tests except the focused test.