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>

tests/cdboot_test.in

@@ -34,6 +34,7 @@ case "${grub_modinfo_target_cpu}-${grub_modinfo_platform}" in
 	exit 0;;
 esac
 
-if [ "$(echo hello | "${grubshell}" --boot=cd)" != "Hello World" ]; then
+v=$(echo hello | "${grubshell}" --boot=cd)
+if [ "$v" != "Hello World" ]; then
    exit 1
 fi