selftests: do not use .ONESHELL

Using one shell for the whole recipe with long lists can cause

make[1]: execvp: /bin/sh: Argument list too long

with some shells. Triggered by commit 309b81f0 ("selftests/bpf:
Install generated test progs")

It requires to change the rule which rely on the one shell
behaviour (run_tests).

Simplify also INSTALL_SINGLE_RULE, remove extra echo, required to
workaround .ONESHELL.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Cc: Jiri Benc <jbenc@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>