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

betterproto/tests/inputs/double/double-negative.json

@@ -0,0 +1,3 @@
+{
+  "count": -123.45
+}

betterproto/tests/inputs/double/double.json

@@ -0,0 +1,3 @@
+{
+  "count": 123.45
+}

betterproto/tests/inputs/double/double.proto

@@ -0,0 +1,5 @@
+syntax = "proto3";
+
+message Test {
+    double count = 1;
+}