[test] Fix TargetParserTest runtime.

r363780 fixes extreme memory growth by using a new std::vector every loop iteration, but causes runtime to go up (and occasionally timeout in certain situations) because of constructor cost every loop iteration. Fix this by moving the constructor back out, but clearing contents in the loop.

Also apply this to the AArch64 features test case, which seems to use the same pattern.

llvm-svn: 363851