Support for specifying a stdin file in an execopts file

This patch modifies sub_test to support specifying the stdin file inside an *execopts file.

Three shootout benchmarks (k-nucleotide, regex-dna, and reverse-complement) use the output of the fasta benchmark as input. 

The size of the input files we need for performance testing is far too large to keep in the repository (50MB to 250MB). A later patch will support the generation and cleanup of these files during testing.

However, once we have these generated files, we would still need to create .stdin files so our test scripts would use them as standard input. Making copies of these files would take up a lot of space, and using links is unattractive. Both might be difficult to clean up.

The solution here is to allow for the stdin file to be specified inside an *execopts file. If the second-to-last argument is '<', then this patch will attempt to use the last argument as the stdin file.

Another benefit of this change is that we can have just one correctness input file per shootout, instead of one per .chpl test.

This change has passed paratest.