selftests/nolibc: export argv0 for some tests

argv0 is the path to nolibc-test program itself, which is a very good
always existing readable file for some tests, let's export it.

Note, the path may be absolute or relative, please make sure the tests
work with both of them. If it is relative, we must make sure the current
path is the one specified by the PWD environment variable.

Suggested-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/lkml/ZKKbS3cwKcHgnGwu@1wt.eu/


Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>