Ensure FSTest uses MockSystemReader

Tests shall not modify ~/.gitconfig. When running tests with bazel this
test failed since bazel isolates tests in a sandbox.

Change-Id: I7dd092afd14972da58a95eb7c200d353f0959fa1
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/util/FSTest.java

 import java.util.concurrent.TimeUnit;
 
 import org.eclipse.jgit.errors.CommandFailedException;
+import org.eclipse.jgit.junit.MockSystemReader;
 import org.eclipse.jgit.junit.RepositoryTestCase;
 import org.eclipse.jgit.lib.RepositoryCache;
 import org.junit.After;
 
 	@Before
 	public void setUp() throws Exception {
+		SystemReader.setInstance(new MockSystemReader());
 		trash = File.createTempFile("tmp_", "");
 		trash.delete();
 		assertTrue("mkdir " + trash, trash.mkdir());