Start using JUnit 5 (#488)

* Start using JUnit 5

This converts a single test class over to JUnit 5 (YamlUtilsTest). The main
differences you'll notice are that @RunWith isn't needed anymore, test classes
and test methods can now be package-private, and the @Test annotation comes from
the org.junit.jupiter.api package instead of org.junit. There's a lot more
differences between 4 and 5 than this that we'll need to keep in mind when
converting more test classes; for some more details, see:
https://www.baeldung.com/junit-5-migration

In order to allow JUnit 4 and 5 test classes to coexist, I've had to add two new
dependencies, org.junit.jupiter:junit-jupiter-engine and
org.junit.vintage:junit-vintage-engine, which exist in addition to junit:junit
for now. Eventually, once we've completed migrating over all JUnit 4 test
classes, then we can remove junit and junit-vintage-engine and just be left with
junit-jupiter-engine.

* Delete no longer needed lockfiles

* Merge branch 'master' into first-junit5

prober/build.gradle

@@ -49,6 +49,8 @@ dependencies {
     testCompile deps['com.google.monitoring-client:contrib']
     testCompile deps['com.google.truth:truth']
     testCompile deps['junit:junit']
+    testCompile deps['org.junit.jupiter:junit-jupiter-engine']
+    testCompile deps['org.junit.vintage:junit-vintage-engine']
     testCompile deps['org.mockito:mockito-core']
     testCompile project(path: ':common', configuration: 'testing')
     testCompile project(path: ':networking', configuration: 'testRuntime')