libstdc++: Allow seeding random engines in testsuite

libstdc++: Allow seeding random engines in testsuite

The testsuite utilities that use random numbers use a
default-constructed mersenne_twister_engine, meaning the values are
reproducable. This adds support for seeding them, controlledby an
environment variable. Defining GLIBCXX_SEED_TEST_RNG=val in the
environment will cause the engines to be seeded with atoi(val) if that
is non-zero, or with a value read from std::random_device otherwise.

Running with different seeds revealed some bugs in the tests, where a
randomly selected iterator was past-the-end (which can't be erased), or
where the randomly populated container was empty, and then we tried to
remove elements from it unconditionally.

libstdc++-v3/ChangeLog:

	* testsuite/util/exception/safety.h (setup_base::generate):
	Support seeding random engine.
	(erase_point, erase_range): Adjust range of random numbers to
	ensure dereferenceable iterators are used where required.
	(generation_prohibited::run): Do not try to erase from empty
	containers.
	* testsuite/util/testsuite_containergen.h (test_containers):
	Support seeding random engine.