Fix tests that try to overwrite part of the signature to make it invalid
Fix tests that try to overwrite part of the signature to make it invalid

They were writing "a" to the beginning of the signature. This will cause
the test to fail if the first character was already "a". This commit
changes them to increment the first byte instead.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

cyli/gotuf,runcom/gotuf,endophage/gotuf,diogomonica/gotuf