Fix shellcheck lint errors

tests/functional/common/functions.sh

@@ -184,7 +184,7 @@ expect() {
     shift
     "$@" && res=0 || res="$?"
     # also match "negative" codes, which wrap around to >127
-    if [[ $res -ne $expected && $res -ne $[256 + expected] ]]; then
+    if [[ $res -ne $expected && $res -ne $((256 + expected)) ]]; then
         echo "Expected exit code '$expected' but got '$res' from command ${*@Q}" >&2
         return 1
     fi
@@ -199,7 +199,7 @@ expectStderr() {
     shift
     "$@" 2>&1 && res=0 || res="$?"
     # also match "negative" codes, which wrap around to >127
-    if [[ $res -ne $expected && $res -ne $[256 + expected] ]]; then
+    if [[ $res -ne $expected && $res -ne $((256 + expected)) ]]; then
         echo "Expected exit code '$expected' but got '$res' from command ${*@Q}" >&2
         return 1
     fi
@@ -287,7 +287,7 @@ checkGrepArgs() {
     for arg in "$@"; do
         if [[ "$arg" != "${arg//$'\n'/_}" ]]; then
             echo "$(callerPrefix)newline not allowed in arguments; grep would try each line individually as if connected by an OR operator" >&2
-            return -101
+            return 155 # = -101 mod 256
         fi
     done
 }