DirCacheCheckoutTest#testCheckoutHierarchy: Add missing call to fail()

Error Prone reports:

  Not calling fail() when expecting an exception masks bugs

See https://errorprone.info/bugpattern/MissingFail

Change-Id: I463510342bb6e6b99b31a0fe264d953340784393
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/DirCacheCheckoutTest.java

@@ -929,6 +929,7 @@ public class DirCacheCheckoutTest extends RepositoryTestCase {
 						"e/g3"));
 		try {
 			checkout();
+			fail("did not throw CheckoutConflictException");
 		} catch (CheckoutConflictException e) {
 			assertWorkDir(mkmap("a", "a", "b/c", "b/c", "d", "d", "e/f",
 					"e/f", "e/g", "e/g3"));