Potentially include file location in error message in `#[should_panic]` tests

[skogseth]

If a test marked with #[should_panic] fails to panic the test will fail, and an error message is returned. The error message in libtest for this case includes the file location: https://github.com/rust-lang/rust/blob/8e0b68e63cd2b7b6d18474fe6f49df6fb1570c25/library/test/src/test_result.rs#L77. This behavior is not mimicked in libtest2 as implemented in #151, but should potentially be.

[epage]

Something we still need to add is the capturing test locations.

[epage]

Created #153 to track that