Finalize and add SD device test

### Basic Infos

- [x] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [x] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [x] I have tested that the issue is present in current master branch (aka latest git).
- [x] I have searched the issue tracker for a similar issue.
- [x] I have filled out all fields below.

#### Platform

- Hardware: All
- Core Version: latest git
- Development Env: All
- Operating System: All

### Problem Description

From #4444 : Sketch below to test SD on device.
Possibly add to devices test, similar to @d-a-v 's interactive WiFi test sketch, maybe add more actions.

### [MCVE](https://stackoverflow.com/help/mcve) Sketch

Steps:

Send m to mount
Send u and y to unmount
Send m to mount again, if SD.end() is commented, you are in a endless "SD initialization failed!" loop.

```cpp

File myFile;
bool mounted = false;

void setup() {
Serial.begin(115200);
}

void loop() {
if (mounted && Serial.peek() == 'u') {
Serial.read();
Serial.println("Unmount SD and press 'y' to continue");
while (Serial.peek() != 'y') {
delay(1000);
}
mounted = false;
Serial.println("SD unmounted.");
Serial.read();
} else if (!mounted && Serial.peek() == 'm') {
Serial.read();

//Uncomment this to finish while (!SD.begin(D2)) endless loop
//SD.end();

while (!SD.begin(D2)) {
Serial.println("SD initialization failed!");
delay(1000);
}
mounted = true;
Serial.println("Initialization succeded!");
}

if (mounted) {
myFile = SD.open("test.txt");
Serial.println("Printing test.txt contents:");
while (myFile.available()) {
Serial.write(myFile.read());
}
myFile.close();
Serial.println();
Serial.println("All contents of test.txt printed!");

Serial.println("SD mounted, press 'u' to unmount");
} else {
Serial.println("SD unmounted, press 'm' to mount");

}

Serial.println("Wait 3 seconds...");
delay(3000);
}
```