Fix the imgdiff failure when running test_common unittest

imgdiff fails on plain strings after we put more checks the input
data. This leads to failure when running unittest
"test_recovery_from_boot". The CL fixes the issue by serving gzipped
dummy images as the test input.

Bug: 68988329
Test: InstallRecoveryScriptFormatTest passes
Change-Id: I1952b311681ac4f39c0e51aef1de1177aebeac43