Remove use of snprintf and simplify

One test case uses snprintf to convert an
int to a string. Change it to use ToString
(which uses a locale-independent version of
std::to_string). Also remove unnecessary
parts of StringContentsSerializer.

test/lint/lint-locale-dependence.py

@@ -45,7 +45,6 @@ from subprocess import check_output, CalledProcessError
 
 KNOWN_VIOLATIONS = [
     "src/dbwrapper.cpp:.*vsnprintf",
-    "src/test/dbwrapper_tests.cpp:.*snprintf",
     "src/test/fuzz/locale.cpp:.*setlocale",
     "src/test/fuzz/string.cpp:.*strtol",
     "src/test/fuzz/string.cpp:.*strtoul",