Allow replacement of test-case assertion on per-fn scale

[luke-biel]

Currently test_case allows replacing it's built in assert via using and with keywords.

We'd like to be able to replace assertion on per-function and global levels. This refers to requirements specified within #31.

Initial work specifies

#[test_case(input2)]
#[test_case_use(assert_nan)]
fn tested(v: X) -> f64 {
	...
}

as possible solution for per-function replacements.