Override int read(byte[], int, int) on test instances of InputStream

This probably doesn't provide any benefit for the tests, but gets
rid of a warning from Error Prone.

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

Change-Id: I584d2e0d18475fad38747b688af6301c423f54a7
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

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

@@ -260,6 +260,12 @@ public class ObjectLoaderTest {
 						fail("never should have reached read");
 						return -1;
 					}
+
+					@Override
+					public int read(byte b[], int off, int len) {
+						fail("never should have reached read");
+						return -1;
+					}
 				};
 			}
 		};

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

@@ -259,6 +259,11 @@ public class UnionInputStreamTest {
 			public int read() throws IOException {
 				throw new IOException("Expected");
 			}
+
+			@Override
+			public int read(byte b[], int off, int len) throws IOException {
+				throw new IOException("Expected");
+			}
 		};
 		@SuppressWarnings("resource" /* java 7 */)
 		final UnionInputStream u = new UnionInputStream(