[libc] Add option to run specific tests

This addition reads command line input to run specific single tests
within a larger call to run all the tests for a particular function.
When the user adds a second argument to the command line, the code skips
all the tests that don't match the user's specified binary. If the user
doesn't specify a test correctly and/or no tests are run, a failure
message prints.

Reviewed By: sivachandra, aeubanks

Differential Revision: https://reviews.llvm.org/D105843