Updated Need For Speed exercise instructions and tests (exercism#2740)

- requiring and testing that `CanFinish` handles the case of battery <
  batteryDrain as `Drive` function

exercises/concept/need-for-speed/.docs/instructions.md

@@ -57,7 +57,7 @@ car = Drive(car)
 
 To finish a race, a car has to be able to drive the race's distance. This means not draining its battery before having crossed the finish line. Implement the `CanFinish` function that takes a `Car` and a `Track` instance as its parameter and returns `true` if the car can finish the race from start to finish; otherwise, return `false`.
 
-Assume that you are currently at the starting line of the race and start the engine of the car for the race. Take into account that the car's battery might not necessarily be fully charged when starting the race:
+Assume that you are currently at the starting line of the race and start the engine of the car for the race. Same as for `Drive` function if `battery` is less than `batteryDrain` car should not move even with partial `speed`. Take into account that the car's battery might not necessarily be fully charged when starting the race:
 ```go
 speed := 5
 batteryDrain := 2

exercises/concept/need-for-speed/need_for_speed_test.go

@@ -266,6 +266,19 @@ func TestCanFinish(t *testing.T) {
 			},
 			expected: false,
 		},
+		{
+			name: "Car cannot finish the track with partial move (part of batteryDrain)",
+			car: Car{
+				speed:        2,
+				batteryDrain: 200,
+				battery:      100,
+				distance:     0,
+			},
+			track: Track{
+				distance: 1,
+			},
+			expected: false,
+		},
 	}
 
 	for _, tt := range tests {