UnionInputStreamTest: Suppress ErrorProne warning about missing synchronized

Error Prone reports that the unsynchronized method skip overrides the
synchronized method in ByteArrayInputStream [1].

This is a test class, so we can just suppress the warning as recommended
in [1].

Note that the suppression causes a warning in Eclipse, because it doesn't
recognize the "UnsynchronizedOverridesSynchronized" as a valid value for
the @SuppressWarnings annotation [2].

[1] https://errorprone.info/bugpattern/UnsynchronizedOverridesSynchronized
[2] https://bugs.eclipse.org/bugs/show_bug.cgi?id=392045

Change-Id: I3e798b448211f1363729091f72fb0ef6a873e599
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/util/io/UnionInputStreamTest.java

@@ -167,6 +167,8 @@ public class UnionInputStreamTest {
 
 		u.add(new ByteArrayInputStream(new byte[] { 20, 30 }) {
 			@Override
+			@SuppressWarnings("UnsynchronizedOverridesSynchronized")
+			// This is only used in tests and is thread-safe
 			public long skip(long n) {
 				return 0;
 			}