Adding tests for cert decoder

[schneim]

Please check the additional tests.

How to access the extensions ?

Looks like the digicert certificate in the test file has some issue for the extensions . I cannot open the extensions in the KeyStore Explorer. Other certs are ok.

Please ignore the intermediate commits from my fork.

Thanks, Markus

[schneim]

Using latest version of KeyStore Explorer 5.4.2, no issue with the digicert.com cert.