Replace the occurrences of Constants.PRODUCT_NAME for a call to
BlazeRuntime#getProductName() or a reference to TestConstants.PRODUCT_NAME for tests.

This CL prepares the codebase in order to delete the constant.

--
MOS_MIGRATED_REVID=122993568
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 cab180b..5c11575 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
@@ -24,6 +24,8 @@
   private TestConstants() {
   }
 
+  public static final String PRODUCT_NAME = "bazel";
+
   /**
    * A list of all embedded binaries that go into the regular Bazel binary.
    */