tests: Add testcases for EMMC targets.

There're two types of targets in applypatch: regular files and EMMC
targets. We have two sets of functions to handle them respectively.

This CL adds testcases to use "EMMC:filename:size:sha1" as the target
name, which triggers the code path for patching EMMC targets.

Bug: 33034669
Test: recovery_component_test passes.

Change-Id: I8f10c6c8d2c1fb083f06a83de91d9e23cb41fb6d