RustTest: specify module name to run exact one test case

[rhysd]

I found one issue for :RustTest.

• some_test.rs

#[test]
fn test_foo() {}

• another_test.rs

#[test]
fn test_foo() {}

Running :RustTest in test_foo() function in some_test.rs runs both test cases some_test::test_foo and another_test::test_foo.

To solve this issue, this patch extracts module name from its file name and specify mod_name::test_func instead of test_func.

[rust-highfive]

r? @steveklabnik

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[da-x]

I wish there was something more precise for this. Opened: rust-lang/cargo#6478

[rhysd]

Looks good. I agree that it's definitely better that cargo supports running a test specified by position in file.
Until the feature is supported, I want to add this patch as the best effort.