chore: add load()s for the Bazel builtin java rules

`load()` statements are being added to prepare for moving the rules out of Bazel and into `rules_java`

gax-java/gax_java.bzl

@@ -26,13 +26,15 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+load("@rules_java//java:defs.bzl", "java_test")
+
 def java_tests(name, srcs, runtime_deps, size):
     classNames = []
     for src in srcs:
         # convert .java file path to fully qualified class name
         className = src[(src.index("/com/") + 1):-5].replace("/", ".")
         classNames.append(className)
-        native.java_test(
+        java_test(
             name = className,
             test_class = className,
             runtime_deps = runtime_deps,