fstests: attempt to fix flaky serve s3 test

Sometimes (particularly on macOS amd64) the serve s3 test fails with
TestIntegration/FsMkdir/FsPutError where it wasn't expecting to get an
object but it did.

This is likely caused by a race between the serve s3 goroutine
deleting the half uploaded file and the fstests code looking for it to
not exist.

This fix treats it like any other eventual consistency problem and
retries the check using the test framework.

fstest/fstests/fstests.go

@@ -762,9 +762,15 @@ func Run(t *testing.T, opt *Opt) {
 			// assert.Nil(t, obj) - FIXME some remotes return the object even on nil
 			assert.NotNil(t, err)
 
+			retry(t, "FsPutError: test object does not exist", func() error {
 				obj, err := f.NewObject(ctx, file2.Path)
+				if err == nil {
+					return fserrors.RetryErrorf("object is present")
+				}
 				assert.Nil(t, obj)
 				assert.Equal(t, fs.ErrorObjectNotFound, err)
+				return nil
+			})
 		})
 
 		t.Run("FsPutZeroLength", func(t *testing.T) {