Let ConfidentialKey & Secret do something from unit tests

[jglick]

Found it frustrating that I could not, for example, call any method on Secret from jshell because ConfidentialStore does not work without Jenkins. Simpler to introduce an in-memory mock for such cases. (DefaultConfidentialStore already has its own test coverage. Leaving ConfidentialStore.TEST to avoid breaking PCT on https://github.com/jenkinsci/credentials-plugin/blob/6f710a7e30156e94aff8a3c6c6799cef04ad35df/src/test/java/jenkins/security/ConfidentialStoreRule.java & https://github.com/jenkinsci/kubernetes-cli-plugin/blob/4f368d86f28a625c8c0c3bf5a4c25304c9d88cfc/src/test/java/jenkins/security/ConfidentialStoreRule.java.)

Proposed changelog entries

• Developer: Secret and ConfidentialKey implementations can now be used from unit tests without JenkinsRule.

[jglick]

Got some test errors in RSAConfidentialKey.getPrivateKey which I cannot reproduce locally, investigating.

[jglick]

Hmm, still seeing CI failures in RSADigitalSignatureConfidentialKeyTest, CryptoConfidentialKeyTest, and RSAConfidentialKeyTest, this time on JDK 11, which I cannot reproduce locally (on either 8 or 11). I have tried to set a predictable RNG seed so I am not getting what is happening here.

[jglick]

Flake, presumably:

java.lang.AssertionError
	at hudson.slaves.PingThreadTest.failedPingThreadResetsComputerChannel(PingThreadTest.java:89)

[timja]

let's give it one more build