tests: fix compile error in matrix_test.go

Error was:

tests/matrix/matrix_test.go:101:9: no new variables on left side of :=

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/matrix/matrix_test.go b/tests/matrix/matrix_test.go
index 65dd691..317158a 100644
--- a/tests/matrix/matrix_test.go
+++ b/tests/matrix/matrix_test.go
@@ -98,7 +98,7 @@ func testWriteN(t *testing.T, fn string, n int) string {
 	}
 
 	d := make([]byte, n)
-	_, err := file.Write(d)
+	_, err = file.Write(d)
 	if err != nil {
 		t.Fatal(err)
 	}