Correct CMake testing

• Use functions rather than macros to avoid problems with variables in conditions (since macro arguments are not variables)
• Conditionally add to file lists and test program lists based upon the configuration options (e.g. JPEG and old-JPEG availability)
• Sync tests, files and option usage with current automake usage

I also added some extra checks to the test runner script, so it will fail immediately if run without a valid command to run, so the original problem reported in #317 (closed) should not be possible to occur in the future if there is a regression elsewhere.

@Su_Laus, please could you give this a try and see if it addresses your concerns?

Closes: #317 (closed)

#358, #359 and #360 created as followups for issues identified while working on this fix.