fix(bazel): do not put temporary test folders next to test files

Currently the tool mappings and temporary environment directories
of integration tests are always put directly into the test files
directory. e.g.

```
<tmp>/.tool-mapping/node
<tmp>/package.json
```

This worked fine in general but some tests might accidentally discover
the `.tool-mapping-X` folder we created. This shouldn't happen as its
an artifact of our integration test infrastructure.

We can fix this by separating these folders into a structure like:

```
<tmp>/.tool-mapping/node
<tmp>/test-sandbox/package.json
```

The test can then just run one folder deeper, in the `test-sandbox`
folder.

bazel/integration/test_runner/runner.ts

@@ -56,14 +56,21 @@ export class TestRunner {
 
   async run() {
     const testTmpDir = await this._getTestTmpDirectoryPath();
-    const testWorkingDir = path.join(testTmpDir, this.testPackageRelativeWorkingDir);
+    const testSandboxDir = path.join(testTmpDir, 'test-sandbox');
+    const testWorkingDir = path.join(testSandboxDir, this.testPackageRelativeWorkingDir);
+
+    // Create the test sandbox directory. The working directory does not need to
+    // be explicitly created here as the test file copying should create the folder.
+    await fs.promises.mkdir(testSandboxDir);
+
     const toolMappings = await this._setupToolMappingsForTest(testTmpDir);
     const testEnv = await this._buildTestProcessEnvironment(testTmpDir, toolMappings.binDir);
 
-    debug(`Copying test fixtures into: ${path.normalize(testTmpDir)}`);
+    debug(`Temporary directory for integration test: ${path.normalize(testTmpDir)}`);
+    debug(`Test files are copied into: ${path.normalize(testSandboxDir)}`);
     console.info(`Running test in directory: ${path.normalize(testWorkingDir)}`);
 
-    await this._copyTestFilesToDirectory(testTmpDir);
+    await this._copyTestFilesToDirectory(testSandboxDir);
     await this._patchPackageJsonIfNeeded(testWorkingDir);
     await this._runTestCommands(testWorkingDir, testEnv);
   }