gdb/testsuite: resolve duplicate test name in gnu_vector.exp

gdb/testsuite: resolve duplicate test name in gnu_vector.exp

While testing on AArch64 I spotted a duplicate test name in the
gdb.base/gnu_vector.exp test.

This commit adds a 'with_test_prefix' to resolve the duplicate.

While I was in the area I updated a 'gdb_test_multiple' call to make
use of $gdb_test_name.

There should be no change in what is tested after this commit.