[scenic][tests] ScenicCtfTests reuse Scenic realm between test cases

This CL creates a persistent Scenic test environment that maintains
connection to the same Scenic realm across test cases within the same
test package. This improvement helps avoid the unnecessary, costly
creation of a new Scenic realm for each test case to prevent testing
timeouts in the future.

Bug: b/454715223
Change-Id: I0e0d25c6e2103c9adf441c3c41635acf56f1f6fd
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1445808
Fuchsia-Auto-Submit: Justin Kobler <jkobler@google.com>
Reviewed-by: Josh Gargus <jjosh@google.com>
Commit-Queue: Justin Kobler <jkobler@google.com>