test: Better handling of stdout and stderr

Git-style tests (test_expect_success etc.) suppress stdout and stderr
unless -v is given. Notmuch-style tests (created by test_begin_subtest
and test_expect_equal) do not have this behavior so implement it the
same.

Additionally, for both test styles, the test-lib.sh is changed so that
the content of suppressed stdout and stderr is shown in case of failed
test.

Finally a test for this functionality is added to basic tests.