Fix Java tests for older Gradle versions

Before this, Gradle 4.4 was required to build the Gradle wrapper and
thus run the Java tests. This was because of all the stuff that had to
be configured when running the wrapper task using the build.gradle file.

Now we use a config file that only specifies the required Gradle version
and nothing else to run the wrapper task. This allows tests to be run
with Gradle versions beginning with 2.

test/wrapper.gradle

@@ -0,0 +1,3 @@
+wrapper {
+    gradleVersion = '4.4.1'
+}