libstdc++: Fix test that fails randomly [PR101866]

This test assumes that the same sequence of three values cannot occur,
which is incorect. It's unlikely, but not impossible.

Perform the check in a loop, so that in the unlikely event of an
identical sequence, we retry. If the library code is buggy it will keep
producing the same sequence and the test will time out. If the code is
working correctly then we will usually break out of the loop after one
iteration, or very rarely after two or three.

libstdc++-v3/ChangeLog:

	PR libstdc++/101866
	* testsuite/experimental/random/randint.cc: Loop and retry if
	reseed() produces the same sequence.