libstdc++: Fix some tests that fail in C++20 mode

The linear_congruential_engine negative tests fail with a different
error in C++20 mode, because double is no longer an invalid type for
NTTP. Adjust the expected errors.

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/random/linear_congruential_engine/requirements/non_uint_neg.cc:
	Adjust expected error for C++20 mode.
	* testsuite/tr1/5_numerical_facilities/random/linear_congruential/requirements/non_uint_neg.cc:
	Likewise.