Set the default workspace name to __main__

See https://github.com/bazelbuild/bazel/wiki/Updating-the-runfiles-tree-structure
for more details about this change.

RELNOTES[INC]: Bazel will now default to using __main__ as the workspace name
(instead of "", as previously) if a project does not provide a workspace
name in the WORKSPACE file. The repository's runfiles will appear under
x.runfiles/__main__/.

--
MOS_MIGRATED_REVID=121376094
diff --git a/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java b/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java
index a14172f..cab180b 100644
--- a/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java
+++ b/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java
@@ -43,7 +43,7 @@
   /**
    * Default workspace name.
    */
-  public static final String WORKSPACE_NAME = "";
+  public static final String WORKSPACE_NAME = "__main__";
 
   /**
    * Name of a class with an INSTANCE field of type AnalysisMock to be used for analysis tests.