test_gen: replace stdlib Json with inline implementation

Towards the goal of removing Json from stdlib, this change replaces
usage of TotallyNotJson in test_gen/gen_abilities with a simple usable
inline implementation of Encoder/DecoderFormatting.

Similarly, the use of TotallyNotJson in test_reporting is not necessary
at all and is replaced with a Decoder that wouldn't actually work, but
which does compile.

crates/compiler/test_gen/Cargo.toml

@@ -45,6 +45,7 @@ roc_reporting = { path = "../../reporting" }
 roc_solve = { path = "../solve" }
 roc_std = { path = "../../roc_std" }
 roc_target = { path = "../roc_target" }
+roc_test_utils = { path = "../../test_utils"}
 roc_types = { path = "../types" }
 roc_unify = { path = "../unify" }
 roc_wasm_interp = { path = "../../wasm_interp" }