fix(@schematics/angular): add warnings and improve Karma config gener… #2291

Job	Run time
list	23s
workflow (devkit-core-tests-rerun)	1m 26s
workflow (modules-builder-tests)	1m 37s
workflow (devkit-core-tests)	1m 21s
workflow (build-unit-tests)	2m 14s
workflow (build-cli)	1m 38s
	8m 39s

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