SshTestBase: Make inner class TestCredentialsProvider static

Change-Id: Id5ba87b24e8a29157d49e9e5b151d65bdbcab37a
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/transport/SshTestBase.java

@@ -392,7 +392,7 @@ public abstract class SshTestBase extends RepositoryTestCase {
 		}
 	}
 
-	private class TestCredentialsProvider extends CredentialsProvider {
+	private static class TestCredentialsProvider extends CredentialsProvider {
 
 		private final List<String> stringStore;