tests: create all-zero gocryptfs.diriv in resetTmpDir()

Tests were failing because this file was missing

integration_tests/example_filesystems_test.go

@@ -15,7 +15,7 @@ const statusTxtContent = "It works!\n"
 func checkStatusTxt(t *testing.T, filename string) {
 	contentBytes, err := ioutil.ReadFile(filename)
 	if err != nil {
-		t.Error(err)
+		t.Fatal(err)
 	}
 	content := string(contentBytes)
 	if content != statusTxtContent {

integration_tests/helpers.go

@@ -38,6 +38,12 @@ func resetTmpDir() {
 		fmt.Println(err)
 		os.Exit(1)
 	}
+	dirIV := make([]byte, 16)
+	err = ioutil.WriteFile(defaultCipherDir + "gocryptfs.diriv", dirIV, 0444)
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
 }
 
 // mount CIPHERDIR "c" on PLAINDIR "p"