Allow for more flexibility in choices of Version for the GraphConcurrencyTest by allowing us to set an initial version, as well as a method to return the version's successor.

--
MOS_MIGRATED_REVID=114367424
diff --git a/src/test/java/com/google/devtools/build/skyframe/InMemoryGraphConcurrencyTest.java b/src/test/java/com/google/devtools/build/skyframe/InMemoryGraphConcurrencyTest.java
index d02200e..28f0da3 100644
--- a/src/test/java/com/google/devtools/build/skyframe/InMemoryGraphConcurrencyTest.java
+++ b/src/test/java/com/google/devtools/build/skyframe/InMemoryGraphConcurrencyTest.java
@@ -13,6 +13,8 @@
 // limitations under the License.
 package com.google.devtools.build.skyframe;
 
+import com.google.devtools.build.lib.util.Preconditions;
+
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
 
@@ -22,6 +24,17 @@
   private ProcessableGraph graph;
 
   @Override
+  protected Version getStartingVersion() {
+    return IntVersion.of(0);
+  }
+
+  @Override
+  protected Version getNextVersion(Version v) {
+    Preconditions.checkState(v instanceof IntVersion);
+    return ((IntVersion) v).next();
+  }
+
+  @Override
   protected void makeGraph() {
     graph = new InMemoryGraph();
   }