testament: `nimout: <empty>` should give error (vacuously true)

[timotheecour]

• followup on testament: nimout now consistently uses nimoutCheck #16189, there should be a doAssert nimout.len > 0 when that testament spec is explicitly set, since nimout uses nimoutCheck which allows subset line matches.
• else, we need to distinguish between nimout vs nimoutExact or equivalent (EDIT: see testament: add nimoutFull: bool spec #17867 which adds nimoutFull: bool)

Example

in tests/stdlib/turi.nim + a few others:

nimout:   ""

Current Output

testament spec accepted

Expected Output

doAssert failure (doAssert is fine if we consider testament + its tests as a whole, otherwise we can also raise an InvalidSpecError if we consider testament + its tests as 2 separate entities for use in library code)

Additional Information

1.5.1 edce589