Add unit test of escape_non_alnum

To show that this function now works correctly with all possible one
byte strings.

Author: thomasspriggs

unit/Makefile

@@ -119,6 +119,7 @@ SRC += analyses/ai/ai.cpp \
        util/string2int.cpp \
        util/structured_data.cpp \
        util/string_utils/capitalize.cpp \
+       util/string_utils/escape_non_alnum.cpp \
        util/string_utils/join_string.cpp \
        util/string_utils/split_string.cpp \
        util/string_utils/strip_string.cpp \