Update TestBase to be Kotlin/Native 1.4 compatible. (#5956)
Update TestBase to be Kotlin/Native 1.4 compatible. (#5956)

* Update TestBase to be Kotlin/Native 1.4 compatible.

Kotlin/Native 1.4 requires overriding `fail(message,cause)` as well.
Using the `override` keyword would break source compatibility with 1.3
so for now this is handled by supressing the virtual member hidden
check.

* Fix indent.

* Update comment location.
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