Adding test for IsDynamicPartition

Adding a test case for checking if flash task is a dynamic partition
flash task. This function is used since is-logical only works in
fastbootd, and should_flash_in_userspace() only works if
$ANDROID_PRODUCT_OUT is set. This function works with fastboot update
when called in bootloader without $OUT dir set

Test: fastboot_test
Change-Id: I65309f97e04fdfc449e61de5cd3a6feff18bc9ab