tests: Do not occlude subshell error codes when used as input to the test command

tests: Do not occlude subshell error codes when used as input to the test command

When using the output of a subshell as input, its error code is ignored in
the context of "set -e". Many test scripts use grub-shell in a subshell with
output used as an argument to the test command to test for expected output.
Refactor these tests so that the subshell output goes to a shell variable,
so that if the subshell errors the script will immediately exit with an
error code.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>