Reuse M123 during fan selftest to report RPM

M123 reports both fan speeds nicely in RPM, as well as the requested PWM
for each so that we don't need to show the test "number" to distinguish
between the two in the serial output.

Firmware/ultralcd.cpp

@@ -7586,7 +7586,7 @@ static void lcd_selftest_setfan(uint8_t speed) {
 
 // Wait for the specified number of seconds while displaying some single-character indicator on the
 // screen coordinate col/row, then perform fan measurement
-static void lcd_selftest_measure_fans(uint8_t delay, uint8_t col, uint8_t row, int n) {
+static void lcd_selftest_measure_fans(uint8_t delay, uint8_t col, uint8_t row) {
     // spin-up delay
     static char symbols[] = {'-', '|'};
     static_assert(1000 / sizeof(symbols) * sizeof(symbols) == 1000);
@@ -7605,9 +7605,7 @@ static void lcd_selftest_measure_fans(uint8_t delay, uint8_t col, uint8_t row, i
         delay_keep_alive(100);
     }
 
-    printf_P(PSTR("Test %d:\n"), n);
-    printf_P(PSTR("Print fan speed: %d\n"), fan_speed[1]);
-    printf_P(PSTR("Extr fan speed: %d\n"), fan_speed[0]);
+    gcode_M123();
 }
 
 static FanCheck lcd_selftest_fan_auto(int _fan)
@@ -7622,7 +7620,7 @@ static FanCheck lcd_selftest_fan_auto(int _fan)
 	case 0:
         setExtruderAutoFanState(3); // extruder fan
         lcd_selftest_setfan(0); // print fan off
-        lcd_selftest_measure_fans(2, 18, 2, 1);
+        lcd_selftest_measure_fans(2, 18, 2);
         setExtruderAutoFanState(0); // extruder fan off
 		if (fan_speed[0] < failThr) {
 			return FanCheck::ExtruderFan;
@@ -7634,7 +7632,7 @@ static FanCheck lcd_selftest_fan_auto(int _fan)
 
 	case 1:
         lcd_selftest_setfan(255);
-        lcd_selftest_measure_fans(5, 18, 3, 2);
+        lcd_selftest_measure_fans(5, 18, 3);
         lcd_selftest_setfan(0);
         if (fan_speed[1] < failThr) {
             return FanCheck::PrintFan;