Parallel: forbid beforeEach/afterEach at the top level of spec files

Each spec file is only loaded in a single worker, so top level
before/afterEach can't behave consistently.

beforeEach/afterEach are still supported in:
* Helper files
* describe() blocks
* At the top level of spec files in non-parallel mode

src/core/SuiteBuilder.js

@@ -22,6 +22,10 @@ getJasmineRequireObj().SuiteBuilder = function(j$) {
       this.focusedRunables = [];
     }
 
+    inDescribe() {
+      return this.currentDeclarationSuite_ !== this.topSuite;
+    }
+
     parallelReset() {
       this.topSuite.removeChildren();
       this.topSuite.reset();