testsuite: use a section to put tests in instead of array

testsuite: use a section to put tests in instead of array

Intead of having to declare an array of tests, tweak the definition of
DEFINE_TEST and TESTSUITE_MAIN so they know the tests are put in a
particular section of the ELF file.

This avoids the mistake of adding a test and forgetting to add it to the
array. Now once a test is defined, it's ready to run, so one less step
to define new tests.

The removal of the arrays is left for another patch so not to clutter
the diff on this one.