I have a 2070 Super FTW3 ULTRA+ and i've overclocked it. I wanna test the stability of overclock. I'm clicking to Test button after awhile when test is completed, im getting a score. But neither Pass nor Fail button lights up at the end of the test. How can i understand that my overclock is stable or unstable ?

no idea why your "Pass nor Fail button lights up at the end of the test"  -- does a + value show up for the GPU ?
 
if the result is something like +50 ... then that is the highest stable value for the GPU that the test found & can be entered for the GPU
 
Just run one of the many free benchmark software packs to push your hardware .. make small adjustments & test .. you may be able to get more out of your GPU

jeffogu
The result was +90. Is this max stable value for stock voltage or including for max voltage ? 

Correct
 
That is the "Value" that the Nvidia Algorithm has established as the Max stable GPU setting for OC.
 
Any more that that would require manual testing & possibly more Voltage or better cooling.   This does Not test the RAM for stability or OC settings - that is still a manual process.
 
Every load is different & synthetic performance is a general performance.  The stability of any Game of other Program can possible be higher or Lower.
 
I test manually for Max stable OC & then reduce that number by 10% ... to increase stability
 
In benchmark runs to establish the absolute Max performance numbers, you might tolerate crashes ... daily PC use & Gaming not so much