Allow for parallel expansion test execution

[eupn]

Closes #44.

[taiki-e]

This seems to mean that every time macrotest::expand is called, all dependencies are re-compiled. For example, the following code will re-compile the dependencies three times, right?

#[test]
fn test() {
    macrotest::expand("tests/expand/a.rs");
    macrotest::expand("tests/expand/b.rs");
    macrotest::expand("tests/expand/c.rs");
}

If so, It may significantly increase the test time. (this is probably an unavoidable cost if the test is may conflicts)
Perhaps it seems preferable to adjust the documentation to recommend using wildcard(*) or expose a type like trybuild::TestCases.

[eupn]

Makes sense! Yes, I'm afraid that dependencies will be recompiled for each set. I'll update the docs.