fix(@schematics/angular): remove async test helper function from component schematic

[clydin]

jasmine has supported promises and async/await natively since 2.7.
The only usage of the async helper function is currently within the beforeEach of the component test schematic but async/await provides equivalent functionality in this context without the need for an additional Zone.js layer.
While the call to compileComponents is typically unneeded when using ng test, the generated test files may be also be used outside the command.

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