This issue mostly occurs when the test times out before it is actually completed. The default timeout used for a test to run is 30 seconds, here your sleep durations add up to more than 30 seconds, hence the connection closed with pending requests.
I don't understand why to sleep for a certain duration after each action, may be you can reduce/remove sleep. If sleeps are mandatory for your case try to pass the appropriate timeout to the test.
Example:
test(
'test main card settings items',
() async {
final cardSettingsItem = find.byValueKey('settings.cardSettings');
final mainSettingsList = find.byValueKey('mainSettingsList');
final profileButton = find.byValueKey('Profile');
final changePinButton = find.byValueKey('Cambiar pin');
final changePinInputField0 = find.byValueKey('0');
final changePinInputField1 = find.byValueKey('1');
final changePinInputField2 = find.byValueKey('2');
final changePinInputField3 = find.byValueKey('3');
final changePinInputField4 = find.byValueKey('4');
final changePinInputField5 = find.byValueKey('5');
final changePinScreenButton = find.byValueKey('changePinScreenButton');
sleep(Duration(seconds: 3));
await driver.tap(profileButton);
sleep(Duration(seconds: 2));
await driver.scrollIntoView(mainSettingsList);
sleep(Duration(seconds: 3));
await driver.tap(cardSettingsItem);
sleep(Duration(seconds: 3));
await driver.tap(changePinButton);
sleep(Duration(seconds: 4));
await driver.tap(changePinInputField0);
sleep(Duration(seconds: 2));
await driver.enterText("0");
sleep(Duration(seconds: 1));
await driver.tap(changePinInputField1);
sleep(Duration(seconds: 2));
await driver.enterText("1");
sleep(Duration(seconds: 1));
await driver.tap(changePinInputField2);
sleep(Duration(seconds: 2));
await driver.enterText("2");
sleep(Duration(seconds: 1));
await driver.tap(changePinInputField3);
sleep(Duration(seconds: 2));
await driver.enterText("3");
sleep(Duration(seconds: 1));
await driver.tap(changePinInputField4);
sleep(Duration(seconds: 2));
await driver.enterText("4");
sleep(Duration(seconds: 4));
await driver.tap(changePinInputField5);
sleep(Duration(seconds: 1));
await driver.enterText("5");
sleep(Duration(seconds: 5));
await driver.tap(changePinScreenButton);
sleep(Duration(seconds: 4));
},
timeout: Timeout(
Duration(minutes: 2),
),
);
Hope that helps!