Document how to select / deselect test group numbers

 -xxx           Removes 'xxx' from the current set of tests.  If this is the

                first token in the list, the current set of tests is first

                assigned the whole set of available tests, effectively making

                this token equivalent to TESTS="alltests -xxx"

                this token equivalent to TESTS="alltests -xxx".

 nn             Adds the test group 'nn' (which is a number) to the current

                set of tests.

 -nn            Removes the test group 'nn' from the current set of tests.

                If this is the first token in the list, the current set of

                tests is first assigned the whole set of available tests,

                effectively making this token equivalent to

                TESTS="alltests -xxx".

 Also, all tokens except for "alltests" may have wildcards, such as *.

 (on Unix and Windows, BSD style wildcards are supported, while on VMS,

 $ make TESTS='test_ssl* -test_ssl_*' test

 Example: Only test group 10:

 $ make TESTS='10'

 Example: All tests except the slow group (group 99):

 $ make TESTS='-99'

 Example: All tests in test groups 80 to 99 except for tests in group 90:

 $ make TESTS='[89]? -90'

 Note on multi-threading

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