tests: Use sequential test runner if only one test is given.
* tests/openpgp/run-tests.scm: Use sequential test runner if only one test is given.
This allows one to set the environment variable TESTFLAGS to
'--parallel' and enjoy faster test execution times without interfering
with stdio when one works on a single test.
- Signed-off-by: Justus Winter <justus@g10code.com>