selftests/bpf: Make a copy of subtest name

selftests/bpf: Make a copy of subtest name

BugLink: https://bugs.launchpad.net/bugs/1858428
[ Upstream commit f90415e9600c5227131531c0ed11514a2d3bbe62 ]

test_progs never created a copy of subtest name, rather just stored
pointer to whatever string test provided. This is bad as that string
might be freed or modified by the end of subtest. Fix this by creating
a copy of given subtest name when subtest starts.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191021033902.3856966-6-andriin@fb.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>