Fix warning about possible future breakage

Incoming Unicode regular expressions allow brackets inside brackets, therefore POSIX semantics no longer apply. Python warns about possible future breakage, so add backslash escape to square bracket opening inside the character class.

unit_tests/expr.suite/llfrand-1.out

@@ -1,2 +1,2 @@
 REGEX
-[[,] [0-9]{7}\.[0-9]*
+[\[,] [0-9]{7}\.[0-9]*

unit_tests/expr.suite/llfrand-2.out

@@ -1,2 +1,2 @@
 REGEX
-[[,] [0-9]{8}\.[0-9]*
+[\[,] [0-9]{8}\.[0-9]*