build: Add a --show-duration option to the SCM test-driver.

* build-aux/test-driver.scm (script-version): Update.
(show-help): Document it.
(%options): Add the 'show-duration' option.
(test-runner-gnu): Pass as a new argument.
[test-cases-start-time]: New inner variable.
[test-on-test-begin-gnu]: New hook, used to record the start time.
[test-on-test-end-gnu]: Conditionally print elapsed time.  Record it as the
optional metadata in the test result file (.trs).
* doc/guix.texi (Running the Test Suite): Document it.