Adds TEST_BINARY (#6291)

## What's the problem this PR addresses?

I want to run the tests on an alternate Yarn build.

## How did you fix it?

Replaces the TEST_FROM_SOURCES environment variable by a more flexible
TEST_BINARY which lets us select any binary on disk to run the tests.

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packages/acceptance-tests/pkg-tests-core/sources/utils/makeTemporaryEnv.ts

@@ -27,9 +27,8 @@ const mte = generatePkgDriver({
       ? [projectFolder]
       : [];
 
-    const yarnBinary = process.env.TEST_FROM_SOURCES
-      ? require.resolve(`${__dirname}/../../../../../scripts/run-yarn.js`)
-      : require.resolve(`${__dirname}/../../../../yarnpkg-cli/bundles/yarn.js`);
+    const yarnBinary = process.env.TEST_BINARY
+      ?? require.resolve(`${__dirname}/../../../../yarnpkg-cli/bundles/yarn.js`);
 
     const res = await execFile(process.execPath, [...execArgv, yarnBinary, ...cwdArgs, command, ...args], {
       cwd: cwd || path,