Add 3 tests: constants and purging modules

These tests check if the use of constants creates dangling pointers
after a module is purged.  They are intended to verify that the
sharing-preserving implementation (when the flag is enabled) handles
constants properly.  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]).