Support multiple test spec arguments. If at least one test fails, the

script exits with a non-zero code (but possibly not immediately, see
--keep-going).

Each test spec is a test config file name or a well-known config name
(no path or extension!). If no specs are given, all known test specs are
used (as before). The same happens if the only test spec given is 'all'.
The following are now equivalent:

    ./test-builds.sh
    ./test-builds.sh all
    ./test-builds.sh btlayer-00-default btlayer-01-minimal btlayer-02-maximus
    ./test-builds.sh test-suite/buildtests/layer-*

You can mix file names and spec names, but not the 'all' macro: There is
currently no support for using 'all' together with other test cases.

Tolerate individual test errors if --keep-going is specified. This helps
when one wants to find more errors than just the first one, especially
when tests are long and are running without a human watching.

When detecting test failures, rely on test-suite/buildtest.sh exit
status code rather than on the presence of error-like strings in the log
file.

Added and polished comments. Comments need more work.