test: run yarn prepare before running tests

This is required because the test projects contain webpack config files that import the parent
project, so the parent project must be built first. Since migrating to yarn 3, the project is not
necessarily built (and does not otherwise need to be built) before tests are run.

Author: djcsdy

index.test.ts

@@ -2,6 +2,10 @@ import {fork} from "child_process";
 import {resolve} from "path";
 import test from "ava";
 
+test.before(async () => {
+    await yarn(".", ["prepare"]);
+})
+
 async function buildTestProject(path: string): Promise<void> {
     return yarn(path, [])
         .then(code => (code === 0 ? yarn(path, ["build"]) : code))