Gets passed to run-tests --list-tests to get actual list of tests to use.
If --target= is not given, legacy set hardcoded in tinytest-codegen itself
is used.

Also, get rid of tinytest test groups - they aren't really used for
anything, and only complicate processing. Besides, one of the next
step is to limit number of tests per a generated file to control
the binary size, which also will require "flat" list of tests.