build: test-driver.scm: Add a new '--errors-only' option.

build: test-driver.scm: Add a new '--errors-only' option.

* build-aux/test-driver.scm (show-help): Add the help text for the
new '--errors-only' option.
(%options): Add the errors-only option.
(test-runner-gnu): Add the errors-only? parameter and update doc.  Move the
logging of the test data after the test has completed, so a choice can be made
whether to keep it or discard it based on the value of the test result.
(main): Pass the errors-only? option to the driver.
* doc/guix.texi (Running the Test Suite): Document the new option.