Organize test-cases into folders, extract compatibility test into proper test, support adding test-case specific tests

betterproto/tests/inputs/casing/casing.json

@@ -0,0 +1,4 @@
+{
+  "camelCase": 1,
+  "snakeCase": "ONE"
+}

betterproto/tests/inputs/casing/casing.proto

@@ -0,0 +1,17 @@
+syntax = "proto3";
+
+enum my_enum {
+  ZERO = 0;
+  ONE = 1;
+  TWO = 2;
+}
+
+message Test {
+  int32 camelCase = 1;
+  my_enum snake_case = 2;
+  snake_case_message snake_case_message = 3;
+}
+
+message snake_case_message {
+
+}