Add 3 tests: constants and purging modules

These tests are intended to detect bugs with the sharing-preserving flag.
They should also pass without problems if the flag is disabled.

    ts:run(kernel, code_SUITE, mod_const_direct, [batch]).
    ts:run(kernel, code_SUITE, mod_const_send, [batch]).
    ts:run(kernel, code_SUITE, mod_const_spawn, [batch]).