style: format code with Go fmt and Gofumpt (#173)

This commit fixes the style issues introduced in 937f93c according to the output
from Go fmt and Gofumpt.

Details: None

Co-authored-by: deepsource-autofix[bot] <62050782+deepsource-autofix[bot]@users.noreply.github.com>
Co-authored-by: Jonathan Schweder <jonathanschweder@gmail.com>

iban_test.go

@@ -124,7 +124,7 @@ func BenchmarkBban(b *testing.B) {
 }
 
 func TestIbanChecksum(t *testing.T) {
-	var ibans = map[string]string{
+	ibans := map[string]string{
 		"AL00212110090000000235698741":        "47",
 		"AD0000012030200359100100":            "12",
 		"AT001904300234573201":                "61",