Create tests to validate PAR file output

[LegalizeAdulthood]

PAR file generation with the makepar command doesn't have any validation tests around it right now.

Experiments with DOS FRACTINT and makepar reveals that it would output #IND (indeterminate floating-point number) for old GIFs.

Validate that PAR file output is correct for images; this would be an end-to-end integration test.

Validate that PAR file output is correct based on variable state. This could be a unit test that validates against a string. The idea here is to validate that variables in their default state don't generate a parameter in the output string.