test: move coverage on ParseNonRFCJSONValue() to UniValue::read()

Preparation to deprecate ParseNonRFCJSONValue() but keep test coverage
on the underlying UniValue::read() unaffected. The test coverage on
AmountFromValue is no longer included, since that is already tested
in the rpc_parse_monetary_values test case.

Fuzzing coverage on ParseNonRFCJSONValue() was duplicated between string.cpp
and parse_univalue.cpp, only the one in parse_univalue.cpp is kept.

src/test/fuzz/parse_univalue.cpp

@@ -21,12 +21,9 @@ FUZZ_TARGET_INIT(parse_univalue, initialize_parse_univalue)
     const std::string random_string(buffer.begin(), buffer.end());
     bool valid = true;
     const UniValue univalue = [&] {
-        try {
-            return ParseNonRFCJSONValue(random_string);
-        } catch (const std::runtime_error&) {
-            valid = false;
-            return UniValue{};
-        }
+        UniValue uv;
+        if (!uv.read(random_string)) valid = false;
+        return valid ? uv : UniValue{};
     }();
     if (!valid) {
         return;