Support "flaky" field on kt_jvm_test

[cgruber]

java_test supports a "flaky=True" flag which causes the test to be executed possibly a number of times, in order to see if it ever passes. It's not a recommended practice, but it does get used to resolve some inherently flaky test situations, especially with legacy tests. Right now, not having it forces rules_kotlin users to not use kt_jvm_test, instead using kt_jvm_library+java_test to do the execution.