Use JUnit 5 for unit test execution (#3620)

Change the unit test execution to use the Junit5 JUnitPlatform. This
allows for a mix of junit 4 and junit 5 tests and for a gradual
migration to junit 5 instead of a big bang. One class depended on
junit 4 exceptions and was updated. Two tests depending on 
native libraries fail gracefully on mac (and only mac).

Signed-off-by: Danno Ferrin <danno.ferrin@gmail.com>

enclave/build.gradle

@@ -16,6 +16,9 @@ dependencies {
 
   testImplementation 'junit:junit'
   testImplementation 'org.assertj:assertj-core'
+  testImplementation 'org.junit.jupiter:junit-jupiter'
+
+  testRuntimeOnly 'org.junit.vintage:junit-vintage-engine'
 
   // integration test dependencies.
   integrationTestImplementation project(':testutil')