Parametrize the JDK used for testing (#10)

Demonstrates using the [`parametrize` builtin](https://www.pantsbuild.org/v2.11/docs/targets#parametrizing-targets) with the [`jdk` field](https://www.pantsbuild.org/v2.11/docs/reference-scalatest_tests#codejdkcode), to run the same test under multiple JDKs.

Author: stuhood

tests/jvm/org/pantsbuild/example/lib/BUILD

@@ -1 +1,5 @@
-scalatest_tests()
+scalatest_tests(
+    # These tests are `parametrize`d, so that they will run against two different
+    # JDK versions.
+    jdk=parametrize(adopt_11="adopt:1.11", adopt_12="adopt:1.12.0.2"),
+)