selftests/bpf: inline TEST_TAG constants in test_loader.c

selftests/bpf: inline TEST_TAG constants in test_loader.c

After str_has_pfx() refactoring each TEST_TAG_* / TEST_BTF_PATH
constant is used exactly once. Since constant definitions are not
shared between BPF-side bpf_misc.h and userspace side test_loader.c,
there is no need in the additional redirection layer.

Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Reviewed-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260410-selftests-global-tags-ordering-v2-4-c566ec9781bf@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>