libstdc++: Suppress more vstring testsuite warnings. [PR 98613]

libstdc++: Suppress more vstring testsuite warnings. [PR 98613]

    PR c++/57111 - 57111 - Generalize -Wfree-nonheap-object to delete

can create false positive warnings for vstring _S_empty_rep.

This patch prunes the excess false positive warnings from two more
testcases.

libstdc++-v3/ChangeLog:

	PR libstdc++/98613
	* testsuite/ext/vstring/cons/moveable.cc: Suppress false positive
	warning.
	* testsuite/ext/vstring/modifiers/assign/move_assign.cc: Same.