Fix shellcheck warnings in the test scripts

Making them more robust to changes.
We ought to automate this some day.

My version of ShellCheck (v0.9.0) errors on test/gfx/test.sh, though...

test/fix/test.sh

@@ -7,8 +7,10 @@ src="$PWD"
 rc=0
 
 cp ../../{rgbfix,contrib/gbdiff.bash} "$tmpdir"
-cd "$tmpdir"
-trap "cd; rm -rf '$tmpdir'" EXIT
+cd "$tmpdir" || exit
+# Immediate expansion is the desired behaviour.
+# shellcheck disable=SC2064
+trap "cd; rm -rf ${tmpdir@Q}" EXIT
 
 bold="$(tput bold)"
 resbold="$(tput sgr0)"
@@ -43,7 +45,7 @@ runTest () {
 		fi
 		if [[ -z "$variant" ]]; then
 			cp "$2/$1.bin" out.gb
-			if [[ -n "$(eval $RGBFIX $flags out.gb '2>out.err')" ]]; then
+			if [[ -n "$(eval "$RGBFIX" $flags out.gb '2>out.err')" ]]; then
 				echo "${bold}${red}Fixing $1 in-place shouldn't output anything on stdout!${rescolors}${resbold}"
 				our_rc=1
 			fi
@@ -52,18 +54,19 @@ runTest () {
 			# Stop! This is not a Useless Use Of Cat. Using cat instead of
 			# stdin redirection makes the input an unseekable pipe - a scenario
 			# that's harder to deal with.
+			# shellcheck disable=SC2002
 			cat "$2/$1.bin" | eval $RGBFIX "$flags" - '>out.gb' '2>out.err'
 			subst='<stdin>'
 		fi
 
 		sed "s/$subst/<filename>/g" "out.err" | tryDiff "$2/$1.err" - "$1.err${variant}"
-		our_rc=$(($? || $our_rc))
+		(( our_rc = our_rc || $? ))
 		if [[ -r "$2/$1.gb" ]]; then
 			tryCmp "$2/$1.gb" "out.gb" "$1.gb${variant}"
-			our_rc=$(($? || $our_rc))
+			(( our_rc = our_rc || $? ))
 		fi
 
-		rc=$(($rc || $our_rc))
+		(( rc = rc || our_rc ))
 		if [[ $our_rc -ne 0 ]]; then break; fi
 	done
 }