Add availability guards to `Duration` conversion tests. (#1746) #1617

Job	Run time
sanitizer_testing (address, debug)	2m 40s
core (6.0.3-noble, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	10m 16s
sanitizer_testing (address, release)	5m 23s
Api Breakage Compared to main branch	0s
core (5.10.1-noble, ref/heads/main)	11m 35s
swift-format Check	0s
sanitizer_testing (thread, debug)	2m 46s
core (5.9.2-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	9m 47s
sanitizer_testing (thread, release)	4m 48s
fuzzing_regressions (debug)	3m 40s
core (5.8.1-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	8m 40s
fuzzing_regressions (release)	3m 3s
	1h 2m 38s

Provide feedback