repr testing: Change datum creation to row creation to get rid of lifetime problem.

[wangandi]

As mentioned in #7676,

pub fn get_datum_from_str<'a>(litval: &'a str, littyp: &ScalarType) -> Result<Datum<'a>, String>

has been changed to

pub fn test_spec_to_row<'a, I>(datum_iter: I) -> Result<Row, String>

to get rid of the lifetime issue associated with creating a datum.

I also moved some helpful code from expr-test-util to repr-test-util:

• If you have a test of the form datum scalar_type, extract_literal_string should automatically parse the datum part out.
• parse_vec_of_literals allows parsing [vec of data] into Vec<String>. You can then create a Row if you zip the Vec<String> with a Vec`.

[danhhz]

LGTM, thanks! Sorry this took a bit, I've been OOO since you sent it

[danhhz]

Did you want to add another repr_test_tuil/tests/testdata file with some examples of rows? Those have been really useful for me in figuring out what the input to these functions looks like

[wangandi]

Sure! I added src/repr-test-util/tests/testdata/row.