tests/run-tests.sh: Remove obsolete comment

There is no such section in README.rst since 7e65d98e.
tests/run-tests.sh accepts testsuite as argument

A "test suite" is the name of a file under "tests/" like
"test-motion.sh" or "test-stbt-run.sh". If you give such a test suite as
an argument, run-tests.sh will run all the test cases inside that test
suite.

This is useful when I'm working on template match, so that after each
change I can say "tests/run-tests.sh test-match.sh" and avoid running
the (quite slow!) tests for the runner, lirc, etc.

You can't mix testsuite names and testcase names in the same command
line.
tests/run-tests.sh: Blank line after failed test log

So that it's easier to see where the failed test's log ends, and the
next test begins.
tests: Remove unnecessary "consecutive_matches"

This was originally set to "consecutive_matches=24" so that each test,
with its match rectangle drawn over the videotestsrc pattern, would
remain visible for 2 seconds to give some reassurance to the human
watching the self-tests run.

Now that the selftests use "fakesink" instead of "ximagesink" (cf2b39dc)
this is no longer necessary, and it slows down the test. By now we have
great confidence in our automated tests so the visual confirmation isn't
necessary.

I left "consecutive_matches=2" in one of the tests (test_wait_for_match)
so that we have at least one place that tests that parameter. (I suppose
a better test would be to use "videotestsrc pattern=ball" and assert
that we don't find a match with consecutive_frames > 1.)
tests/run-performance-test.sh: Cleanup to allow testing "stbt run -vv"

This commit doesn't change the behaviour or performance measurements of
the script. It simply allows the script to work even if you manually
edit the script to pass "-vv" to "stbt run".

The pattern "/ Position/" has a space on purpose, so that it only
matches the output of the python script's "print" statement, and not the
debug output from "stbt run" itself, which looks like
"..., position=Position...".
stbt.MatchParameters: Check value of `match_method` and `confirm_method`

These parameters are strings, so it is easy to make a typo. In the case
of `confirm_method`, unexpected values were being treated as "absdiff".