tests: shellcheck misc

Few more minor complains from ShellCheck.

test/lib/check.sh

@@ -240,10 +240,10 @@ in_sync() {
 	fi
 
 	if [[ ${a[$(( idx - 1 ))]} =~ a ]] ; then
-		[ $ignore_a -eq 0 ] && \
+		[ "$ignore_a" = 0 ] && \
 			die "$lvm_name ($type$snap) in-sync, but 'a' characters in health status"
 		echo "$lvm_name ($type$snap) is not in-sync"
-		[ $ignore_a -eq 1 ] && return 0
+		[ "$ignore_a" = 1 ] && return 0
 		return 1
 	fi
 

test/lib/utils.sh

@@ -39,7 +39,7 @@ rand_bytes() {
 
 	# Ensure that $data has length at least 50+$n
 	while :; do
-		len=$(echo "$data" | wc -c)
+		len=${#data} # number of chars in $data
 		test "$n_plus_50" -le "$len" && break;
 		data=$( (echo "$data"; eval "$cmds") 2>&1 | gzip )
 	done
@@ -112,7 +112,7 @@ stacktrace() {
 	# i=1 - ignoring innermost frame - it is always stacktrace function
 	local i=1 n=${#BASH_LINENO[*]}
 	# n-=1 - ignoring last frame as well - it is not interesting
-	let n-=1
+	n=$(( n - 1 ))
 
 	echo "## - $0:${BASH_LINENO[$((n-1))]}"
 	while [[ $i -lt $n ]]; do