test_vendor: Add Iterator tests

Add Iterator tests to check that the overloaded operators and bounded iterators
are implemented correctly.

Test: Run the unit tests:
out/host/linux-x86/nativetest64/test-vendor_test_host/\
test-vendor_test_host --gtest_filter=IteratorTest.*
Change-Id: I6402cc36fe6818fa972524ac20aed526128a8553