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

betterproto/tests/inputs/repeatedpacked/repeatedpacked.json

@@ -0,0 +1,5 @@
+{
+  "counts": [1, 2, -1, -2],
+  "signed": ["1", "2", "-1", "-2"],
+  "fixed": [1.0, 2.7, 3.4]
+}

betterproto/tests/inputs/repeatedpacked/repeatedpacked.proto

@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+message Test {
+    repeated int32 counts = 1;
+    repeated sint64 signed = 2;
+    repeated double fixed = 3;
+}