SitePathsTest: Convert to use Google Truth

Convert the tests to use Google Truth instead of junit.Assert.

Add a new PathSubject to perform assertions about Path objects.

Change-Id: I8016daeebc6f8cb291f9a767441f5e1a52ea5097

gerrit-test-util/src/main/java/com/google/gerrit/extensions/common/PathSubject.java

@@ -0,0 +1,40 @@
+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.extensions.common;
+
+import static com.google.common.truth.Truth.assertAbout;
+
+import com.google.common.truth.FailureStrategy;
+import com.google.common.truth.Subject;
+import com.google.common.truth.SubjectFactory;
+import java.nio.file.Path;
+
+public class PathSubject extends Subject<PathSubject, Path> {
+  private static final SubjectFactory<PathSubject, Path> PATH_SUBJECT_FACTORY =
+      new SubjectFactory<PathSubject, Path>() {
+        @Override
+        public PathSubject getSubject(FailureStrategy failureStrategy, Path path) {
+          return new PathSubject(failureStrategy, path);
+        }
+      };
+
+  private PathSubject(FailureStrategy failureStrategy, Path path) {
+    super(failureStrategy, path);
+  }
+
+  public static PathSubject assertThat(Path path) {
+    return assertAbout(PATH_SUBJECT_FACTORY).that(path);
+  }
+}