This doesn't verify that they're atomic, but does verify that they
return the right things.

This commit adds a new test function _dbus_test_check (a) to make
writing tests easier. It checks the given boolean expression and
generates a "not ok" test result if the expression is false.

Due to the current design of the test api, the test is only compiled
if embedded tests were enabled at the time of configuration.

It was also necessary to move the test_atomic target definitions in
test/Makefile.am to the --enable-embedded-tests section to avoid a
make distcheck build error.

The test case itself has been authored by smcv.

Co-authored-by: Simon McVittie <smcv@collabora.com>