test-parse-options: --expect=<string> option to simplify tests

test-parse-options: --expect=<string> option to simplify tests

Existing tests in t0040 follow a rather verbose pattern:

        cat >expect <<\EOF
        boolean: 0
        integer: 0
        magnitude: 0
        timestamp: 0
        string: (not set)
        abbrev: 7
        verbose: 0
        quiet: 3
        dry run: no
        file: (not set)
        EOF

        test_expect_success 'multiple quiet levels' '
                test-parse-options -q -q -q >output 2>output.err &&
                test_must_be_empty output.err &&
                test_cmp expect output
        '

But the only thing this test cares about is if "quiet: 3" is in the
output.  We should be able to write the above 18 lines with just
four lines, like this:

	test_expect_success 'multiple quiet levels' '
		test-parse-options --expect="quiet: 3" -q -q -q
	'

Teach the new --expect=<string> option to test-parse-options helper.

Signed-off-by: Junio C Hamano <gitster@pobox.com>