tap: new `--stderr-prefix' option to prefix test script's stderr

* lib/tap-driver.sh: Add a new `--stderr-prefix' option to instruct
the TAP driver to prefix each line of the test script's standard error
with the given string.  This is useful when `--merge' is enabled and
standard error lines might be confused with test results.
* doc/automake.texi: Document the new `--stderr-prefix' option.
* t/tap-stderr-prefix.tap: New test.
* t/list-of-tests.mk (handwritten_TESTS): Add it.