Copy basic Surefire configuration, esp. forkCount=2, to the plugin POM.

Shaves >11min off the test time for maven-plugin on my machine!

plugins/pom.xml

@@ -31,6 +31,7 @@
 
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    <concurrency>2</concurrency> <!-- may use e.g. 2C for 2 × (number of cores) -->
   </properties>
 
   <dependencies>
@@ -158,7 +159,7 @@
       </plugin>
       <plugin>
         <artifactId>maven-surefire-plugin</artifactId>
-        <!-- version specified in grandparent pom -->
+        <version>2.16</version>
         <configuration>
           <systemProperties>
             <property>
@@ -166,6 +167,9 @@
               <value>33849</value>
             </property>
           </systemProperties>
+          <reuseForks>true</reuseForks>
+          <forkCount>${concurrency}</forkCount>
+          <redirectTestOutputToFile>true</redirectTestOutputToFile>
         </configuration>
       </plugin>
       <plugin>

pom.xml

@@ -348,7 +348,7 @@ THE SOFTWARE.
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-surefire-plugin</artifactId>
-          <version>2.15</version> <!-- ignoring ${maven-surefire-plugin.version} -->
+          <version>2.16</version> <!-- ignoring ${maven-surefire-plugin.version} -->
           <configuration>
             <systemPropertyVariables>
               <java.io.tmpdir>${project.build.directory}</java.io.tmpdir>

test/src/main/java/org/jvnet/hudson/test/WarExploder.java

@@ -94,6 +94,7 @@ final class WarExploder {
         if(!timestamp.exists() || (timestamp.lastModified()!=war.lastModified())) {
             System.out.println("Exploding jenkins.war at "+war);
             new FilePath(explodeDir).deleteRecursive();
+            // TODO this can fail (race condition?) when running tests in parallel; need a three-state flag, and switch to a different explodeDir as needed
             new FilePath(war).unzip(new FilePath(explodeDir));
             if(!explodeDir.exists())    // this is supposed to be impossible, but I'm investigating HUDSON-2605
                 throw new IOException("Failed to explode "+war);