Load TestUtils.java as a library instead of a test source file

The tests of the cfoauth-plugin failed on master branch with 3 failures.
One failure was caused by the TestUtils-class, which did not contain
any method annotated with `@Test`, causing Junit to raise
`java.lang.Exception: No runnable methods tests`.

This change creates a library containing the TestUtils-class, which is
loaded as a test-dependency and excludes TestUtils.java from the
test-sources.

This fixes one of the three observed test failures. The other test
failures will be addressed in a dedicated change.

Change-Id: I393969b46c39757d8bd4c84cda8d3b8a884b7510

BUILD

diff --git a/BUILD b/BUILD
index a28f52c..7a44dca 100644
--- a/BUILD
+++ b/BUILD
@@ -1,15 +1,22 @@
 load("//tools/bzl:junit.bzl", "junit_tests")
 load(
     "//tools/bzl:plugin.bzl",
-    "gerrit_plugin",
     "PLUGIN_DEPS",
     "PLUGIN_TEST_DEPS",
+    "gerrit_plugin",
 )
 
+TEST_SRCS = "src/test/java/**/*Test.java"
+
+TEST_DEPS = PLUGIN_DEPS + PLUGIN_TEST_DEPS + [
+    ":cfoauth__plugin",
+    "@scribe//jar",
+    "@commons_codec//jar",
+]
+
 gerrit_plugin(
     name = "cfoauth",
     srcs = glob(["src/main/java/**/*.java"]),
-    resources = glob(["src/main/resources/**/*"]),
     manifest_entries = [
         "Gerrit-PluginName: cfoauth",
         "Gerrit-Module: com.googlesource.gerrit.plugins.cfoauth.OAuthModule",
@@ -18,19 +25,29 @@
         "Implementation-Title: Cloud Foundry UAA OAuth 2.0 Authentication Provider",
         "Implementation-URL: https://gerrit-review.googlesource.com/#/admin/projects/plugins/cfoauth",
     ],
+    resources = glob(["src/main/resources/**/*"]),
     deps = [
-        "@scribe//jar",
         "@commons_codec//jar:neverlink",
+        "@scribe//jar",
     ],
 )
 
+java_library(
+    name = "testutils",
+    testonly = 1,
+    srcs = glob(
+        include = ["src/test/java/**/*.java"],
+        exclude = [TEST_SRCS],
+    ),
+    deps = TEST_DEPS,
+)
+
 junit_tests(
     name = "cfoauth_tests",
-    srcs = glob(["src/test/java/**/*.java"]),
+    testonly = 1,
+    srcs = glob([TEST_SRCS]),
     tags = ["cfoauth"],
-    deps = PLUGIN_DEPS + PLUGIN_TEST_DEPS + [
-        ":cfoauth__plugin",
-        "@scribe//jar",
-        "@commons_codec//jar",
+    deps = TEST_DEPS + [
+        ":testutils",
     ],
 )