selftests/lkdtm: Avoid needing explicit sub-shell

selftests/lkdtm: Avoid needing explicit sub-shell

Some environments do not set $SHELL when running tests. There's no
need to use $SHELL here anyway, since "cat" can be used to receive any
delivered signals from the kernel. Additionally avoid using bash-isms
in the command, and record stderr for posterity.

Fixes: 46d1a0f03d66 ("selftests/lkdtm: Add tests for LKDTM targets")
Cc: stable@vger.kernel.org
Suggested-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Suggested-by: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210623203936.3151093-2-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>