test: fix flaky controller-manager tests

[karlkfi]

Go Testing doesn't handle panics in goroutines well. So test fatals need to happen from the main thread. But errors can still happen in goroutines. So change the controller-manager stopping to assert instead of require no errors.

[karlkfi]

/retest

Different unit test flaked:

--- FAIL: TestRoot_SourceReconcilerErrorsMetricValidation (0.01s)
    --- FAIL: TestRoot_SourceReconcilerErrorsMetricValidation/single_reconciler_error_in_source_component (0.00s)
        root_test.go:1308: Unexpected metric(s) found:
            + { { {component sync}{errorclass 1xxx} }&{0} }+ { { {component sync}{errorclass 2xxx} }&{0} }+ { { {component sync}{errorclass 9xxx} }&{0} }

[nan-yu]

/lgtm

[google-oss-prow]

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