Add test for missing certificate with AllowMissingDependencies

Add a test that verifies that that missing certificate path inserted
when AllowMissingDependencies is set uses a well formed extension
that won't trip up sign_target_files_apks.

Fixes: 259861670
Test: TestCertificates
Change-Id: I00656809698aed5f9d3aaadaecda9848d2882223