Add make only function to convert strings to lowercase and uppercase.
This can replace the `$(shell echo $(var) | tr 'a-z-' 'A-Z_')` pattern.
Using the 'make' implementation results in being around 100 times faster.
TESTS is used to find the tests files for the application but was used
to list test targets. Rename to COMPILE_TESTS as already done in some of
my other applications.