[flang] Move External IO tests to use GTest

Port external-io test to use GTest. Remove Runtime tests directory.
Rename RuntimeGTest directory to Runtime.

This is the last in a series of patches which ported tests from the old
flang/unittests/Runtime test directory to use GTest in a temporary
unittest directory under flang/unittests/RuntimeGTest. Now that all the
tests in the old directory have been ported to use GTest, the old
directory has been removed and the GTest directory has been renamed to
flang/unittests/Runtime.

Differential Revision: https://reviews.llvm.org/D105315
Reviewed by: Meinersbur, awarzynski