feat(testlab): expose "sandbox.path" property

Allow TestSandbox consumers to access the path of the sandbox directory.

Clean up the code by moving "validateInst" check into "path" getter,
thus avoiding the need to explicitly call "validateInst" in every
public API method.

packages/testlab/src/test-sandbox.ts

@@ -20,7 +20,16 @@ import {
  */
 export class TestSandbox {
   // Path of the TestSandbox
-  private path: string;
+  private _path?: string;
+
+  public get path(): string {
+    if (!this._path) {
+      throw new Error(
+        `TestSandbox instance was deleted. Create a new instance.`,
+      );
+    }
+    return this._path;
+  }
 
   /**
    * Will create a directory if it doesn't already exist. If it exists, you
@@ -30,35 +39,21 @@ export class TestSandbox {
    */
   constructor(path: string) {
     // resolve ensures path is absolute / makes it absolute (relative to cwd())
-    this.path = resolve(path);
+    this._path = resolve(path);
     ensureDirSync(this.path);
   }
 
-  /**
-   * This function ensures a valid instance is being used for operations.
-   */
-  private validateInst() {
-    if (!this.path) {
-      throw new Error(
-        `TestSandbox instance was deleted. Create a new instance.`,
-      );
-    }
-  }
-
   /**
    * Returns the path of the TestSandbox
    */
   getPath(): string {
-    this.validateInst();
     return this.path;
   }
 
   /**
    * Resets the TestSandbox. (Remove all files in it).
    */
   async reset(): Promise<void> {
-    this.validateInst();
-
     // Decache files from require's cache so future tests aren't affected incase
     // a file is recreated in sandbox with the same file name but different
     // contents after resetting the sandbox.
@@ -75,9 +70,8 @@ export class TestSandbox {
    * Deletes the TestSandbox.
    */
   async delete(): Promise<void> {
-    this.validateInst();
     await remove(this.path);
-    delete this.path;
+    delete this._path;
   }
 
   /**
@@ -86,7 +80,6 @@ export class TestSandbox {
    * @param dir Name of directory to create (relative to TestSandbox path)
    */
   async mkdir(dir: string): Promise<void> {
-    this.validateInst();
     await ensureDir(resolve(this.path, dir));
   }
 
@@ -101,7 +94,6 @@ export class TestSandbox {
    * (relative to TestSandbox). Original filename used if not specified.
    */
   async copyFile(src: string, dest?: string): Promise<void> {
-    this.validateInst();
     dest = dest
       ? resolve(this.path, dest)
       : resolve(this.path, parse(src).base);