ThrowableSubject does not provide access to suppressed exceptions

I'm testing a testing library which aside from hurting my brain I have a need to assert on the suppressed exceptions which are coming out attached to the exceptions my library throws.

This is a Java 7 feature, so I'm not sure how you'd go about exposing it without requiring that I do `Truth8.assertThat(throwable)`, but being able to do this is far more useful than how exactly it works. Especially given that I no longer have access to the underlying `Throwable` from the subject to do something like `assertThat(throwable.suppressed[0]).isSameInstanceAs(other)`.