[gdb/testsuite] Fix require dwarf2_support check in some test-cases, some more

The Linaro CI reported a regression in test-case
gdb.dwarf2/macro-source-path-clang14-dw4.exp due to recent commit 81e5a23c7b8
("[gdb/testsuite] Fix require dwarf2_support check in some test-cases").

The problem is that the "require dwarf2_support" in its new location doesn't
work because proc dwarf2_support is not defined.

I didn't notice this because I tested all gdb.dwarf2 test-cases together, and
a different test-case had already imported the proc.

Fix this by moving load_lib dwarf.exp earlier.

Tested on x86_64-linux.