Do you suggest me to test even the micros function itself?
You do that when there is no function between the start and the end of the timing.
In order to test an operation mutiple times, which one of these statements will be faster: for or while?
The for will be the fastest but simply use copy and paste and repeat the single command then there is not any overhead for the loop. You can not use a blank loop to find the overhead because the compiler spots that and optimists it out of existence.
How can I test a substraction operation?
Taken care of in the blank, nothing to test - test.