Replace backticks in shell scripts with S()

tests/008-numeric

@@ -11,8 +11,8 @@ dd if=/dev/zero bs=100 count=1 2>/dev/null \
 # The number of output lines should be >8 and <13, and the final percentage
 # should be 100.
 #
-test `wc -l < $TMP1` -gt 8
-test `wc -l < $TMP1` -lt 13
-test `sed -n '$p' < $TMP1` -eq 100
+test $(wc -l < $TMP1) -gt 8
+test $(wc -l < $TMP1) -lt 13
+test $(sed -n '$p' < $TMP1) -eq 100
 
 # EOF