genTestCerts.go should be more ergonomic when generating data destined for the testdata directory

[christopher-henderson]

• Generating the cert, creating the file by hand, copying the data into the file, and pointing the test to correct file is cumbersome.
• Generating test data for not effective date is painful without access to the util package.
• newLeaf was too far down into the file, leading to excessive scanning up and down.