SF: Replace futureOf with ftl::yield in tests

ftl::yield is equivalent and slightly more efficient as of
I9a7ca7de17e7af10515de97d2f6a0dfa24e35d7a.

Bug: 185536303
Test: CompositionTest
Change-Id: I74c8cffa5c228a07302c8d3665044113c0432a02
1 file changed