Add some basic unit tests for the following files: (#1050)

* Add some basic unit tests for cluster, kube-auth-proxy, and kubeconfig-manager

- src/main/cluster.ts
- src/main/kube-auth-proxy.ts
- src/main/kubeconfig-manager.ts

Signed-off-by: Sebastian Malton <sebastian@malton.name>

package.json

@@ -289,6 +289,7 @@
     "identity-obj-proxy": "^3.0.0",
     "include-media": "^1.4.9",
     "jest": "^26.0.1",
+    "jest-mock-extended": "^1.0.10",
     "make-plural": "^6.2.1",
     "material-design-icons": "^3.0.1",
     "mini-css-extract-plugin": "^0.9.0",