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

dependencies.gradle

@@ -53,8 +53,8 @@ ext {
       'com.google.errorprone:error_prone_annotations:2.3.3',
       'com.google.flogger:flogger-system-backend:0.1',
       'com.google.flogger:flogger:0.1',
-      'com.google.guava:guava-testlib:28.1-jre',
-      'com.google.guava:guava:28.1-jre',
+      'com.google.guava:guava-testlib:28.2-jre',
+      'com.google.guava:guava:28.2-jre',
       'com.google.gwt:gwt-user:2.8.2',
       'com.google.http-client:google-http-client-appengine:1.29.2',
       'com.google.http-client:google-http-client-jackson2:1.29.2',
@@ -98,6 +98,8 @@ ext {
       'jline:jline:1.0',
       'joda-time:joda-time:2.9.2',
       'junit:junit:4.13',
+      'org.junit.jupiter:junit-jupiter-engine:5.6.0',
+      'org.junit.vintage:junit-vintage-engine:5.6.0',
       'org.apache.avro:avro:1.8.2',
       'org.apache.beam:beam-runners-direct-java:2.16.0',
       'org.apache.beam:beam-runners-google-cloud-dataflow-java:2.16.0',