Add a target for heavy tests

[ClementPernet]

In #193 it was noticed that test-ffarith involves a test requiring 1.5Gb causing some debian builds to fail due to memory constraints. Stating that Givaro requires 1.5Gb of RAM seems unreasonnable, hence I suggest to make such heavy tests (either on the computation time or the memory requirements) optional.

• either by adding an option at configure time, setting some macro enabling/disabling such tests in the main testsuite.
• or by introducing a new Makefile target for those heavy tests.