Fix user home config conflicting with the test suite

The test suite did not properly isolate the home config from
influencing the behavior of the tests. Therefore if a user
used the global configuration and also cloned Yarn, the tests could fail.

A primary example of that was when a user set the prefix value.
If the user would set the prefix, the test suite would fail.

__tests__/commands/_helpers.js

@@ -101,12 +101,15 @@ export async function run<T, R>(
 
   // remove the lockfile if we create one and it didn't exist before
   const lockfile = await createLockfile(cwd);
+  const homeFolderLocation = process.env.YARN_HOME_FOLDER;
+  process.env.YARN_HOME_FOLDER = path.join(cwd, '.yarn-home');
 
   // create directories
   await fs.mkdirp(path.join(cwd, '.yarn-global'));
   await fs.mkdirp(path.join(cwd, '.yarn-link'));
   await fs.mkdirp(path.join(cwd, '.yarn-cache'));
   await fs.mkdirp(path.join(cwd, 'node_modules'));
+  await fs.mkdirp(path.join(cwd, '.yarn-home'));
 
   // make sure the cache folder been created in temp folder
   if (flags.cacheFolder) {
@@ -130,5 +133,7 @@ export async function run<T, R>(
     }
   } catch (err) {
     throw new Error(`${err && err.stack} \nConsole output:\n ${out}`);
+  } finally {
+    process.env.YARN_HOME_FOLDER = homeFolderLocation;
   }
 }

src/registries/npm-registry.js

@@ -97,7 +97,7 @@ export default class NpmRegistry extends Registry {
   async getPossibleConfigLocations(filename: string): Promise<Array<[boolean, string, string]>> {
     const possibles = [
       [false, path.join(this.cwd, filename)],
-      [true, this.config.userconfig || path.join(userHome, filename)],
+      [true, this.config.userconfig || path.join(process.env.YARN_HOME_FOLDER || userHome, filename)],
       [false, path.join(getGlobalPrefix(), filename)],
     ];