test: Move the system-test envelope matchers into sentry-test-support

In-module integration tests need the same envelope matchers the sample
system tests use. TestHelper now delegates to the shared copies, so the
sample tests keep working unchanged while they are moved.

Mismatch diagnostics now flush System.out instead of closing it, which
silenced all output after the first mismatch.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: runningcode