Initializing or using CHPL_TEST_TMP_DIR in fasta-dependent shootouts

CHPL_TEST_TMP_DIR, if not specified, will point to a mktemp'd directory for use during testing. It is removed at the end of every test.

Also updated are the fasta-dependent performances scripts that generate files, where the target is now this new temporary directory.
Maybe add a `-f` here to make sure it is deleted.