signer: use a stubbed CT server for tests
signer: use a stubbed CT server for tests

geligaoli/cfssl,sfurman3/cfssl,sfurman3/cfssl,cloudflare/cfssl,sfurman3/cfssl,endocode/cfssl,cloudflare/cfssl,endocode/cfssl,cloudflare/cfssl,cloudflare/cfssl,sfurman3/cfssl,drcapulet/cfssl,drcapulet/cfssl,endocode/cfssl,endocode/cfssl,geligaoli/cfssl,sfurman3/cfssl,geligaoli/cfssl,drcapulet/cfssl,geligaoli/cfssl,geligaoli/cfssl,drcapulet/cfssl,drcapulet/cfssl,endocode/cfssl