unit-test-app: allow specifying subset of tests for configuration

unit-test-app: allow specifying subset of tests for configuration

For some test configurations, not all tests need to be run. This
change allows adding a comment in the configuration file of the
following form:

unit-test-app makefile will use it to determine which components to
build. If the comment is not present, all components will be built
(TESTS_ALL=1).

Also add test configuration for libsodium. It is a separate test
configuration due to the large binary size generated when building
libsodium tests.