refactor(test): rename psbt test files to clarify fixed script handling

Files renamed to better indicate their purpose for fixed script compatibility.

Issue: BTC-1845

Author: OttoAllmendinger

packages/wasm-miniscript/test/psbt.ts renamed to packages/wasm-miniscript/test/psbtFixedScriptCompat.ts

@@ -1,6 +1,6 @@
 import * as utxolib from "@bitgo/utxo-lib";
 import * as assert from "node:assert";
-import { getPsbtFixtures, PsbtStage } from "./psbtFixtures";
+import { getPsbtFixtures, PsbtStage } from "./psbtFixedScriptCompatFixtures";
 import { Descriptor, Psbt } from "../js";
 
 import { getDescriptorForScriptType } from "./descriptorUtil";