Set surefire's tempdir to target

This patch follows up on the review given for revelc#51 and uses
maven-surefire-plugin's configuration to set the tempdir to target and
restore the behavior change introduced in that PR.

Note: This fix will only effect running the test via maven. If the
test is manually run (e.g., from an IDE), java.io.tmpdir should be
overridden manually too.

pom.xml

@@ -435,6 +435,16 @@
           </execution>
         </executions>
       </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-surefire-plugin</artifactId>
+        <version>3.0.0-M5</version>
+        <configuration>
+          <systemPropertyVariables>
+            <java.io.tmpdir>${user.dir}/target</java.io.tmpdir>
+          </systemPropertyVariables>
+        </configuration>
+      </plugin>
     </plugins>
   </build>
   <reporting>