This reduces compilation time, since these are merely linked with the
pre-built module, rather than recompiling it into the test (which
allows for fancy things like failtest).

This reduces the test compile time down from about 62 seconds to 45
seconds.  Since ccanlint compiles tests three times (once normally,
once with coverage, and once with reduced config.h) this makes a
difference: we go from 780 seconds to 729 seconds.