releasetools: Enable a few tests in OtaFromTargetFilesTest.

They used to be disabled due to the assertion of search_path in setUp()
function, which is not a prerequisite for most of the tests.

Bug: 112080715
Test: `atest releasetools_test`
Test: TreeHugger
Change-Id: I3cbaf42aa09dba0b87a64e11d97de9b3f7af7a47