Simplify RetentionTest to use autorelease pools instead of threads.
Simplify RetentionTest to use autorelease pools instead of threads.

PiperOrigin-RevId: 332232200

groschovskiy/j2objc,google/j2objc,google/j2objc,google/j2objc,groschovskiy/j2objc,mirego/j2objc,groschovskiy/j2objc,mirego/j2objc,mirego/j2objc,groschovskiy/j2objc,mirego/j2objc,groschovskiy/j2objc,google/j2objc,mirego/j2objc,google/j2objc,google/j2objc,mirego/j2objc,groschovskiy/j2objc