selftests/exec: Verify execve of non-regular files fail

Add a named pipe as an exec target to make sure that non-regular
files are rejected by execve() with EACCES. This can help verify
commit 73601ea5

 ("fs/open.c: allow opening only regular files
during execve()").

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>