Fix shellcheck SC2006 warnings in 'coverage' script

scripts/test/coverage

@@ -8,19 +8,19 @@
 dst=$1
 
 for fn in src/core/*/*.c src/feature/*/*.c src/app/*/*.c src/lib/*/*.c; do
-    BN=`basename $fn`
-    DN=`dirname $fn`
-    F=`echo $BN | sed -e 's/\.c$//;'`
+    BN=$(basename $fn)
+    DN=$(dirname $fn)
+    F=$(echo $BN | sed -e 's/\.c$//;')
     GC="${BN}.gcov"
     # Figure out the object file names
-    ONS=`echo ${DN}/src_*-${F}.o`
+    ONS=$(echo ${DN}/src_*-${F}.o)
     ONS_WILDCARD_LITERAL="${DN}/src_*-${F}.o"
     # If the wildcard didn't expand, no files
     if [ "$ONS" != "${ONS_WILDCARD_LITERAL}" ]
     then
       for on in $ONS; do
         # We should have a gcno file
-        GCNO=`echo $on | sed -e 's/\.o$/\.gcno/;'`
+        GCNO=$(echo $on | sed -e 's/\.o$/\.gcno/;')
         if [ -e $GCNO ]
         then
           # No need to test for gcda, since gcov assumes no execution