test: add error snippet formatting test

_test_error_snippet/bux.toml

@@ -0,0 +1,4 @@
+[package]
+name = "error_snippet_test"
+version = "0.1.0"
+pkgType = "bin"

_test_error_snippet/src/Main.bux

@@ -0,0 +1,4 @@
+func Main() -> int {
+    let x: int = undefined_variable;
+    return 0;
+}