consistent sharding logic when no shards file

[moshe-autoleadstar]

What:

• Bug Fix
• New Feature

Description:

without the file: 1/3 = a,b,c; 2/3 = d,e,f; 3/3 = g,h,i
with the file (empty): 1/3 = a,d,g; 2/3 = b,e,h; 3/3 = c,f,i
this PR makes the logic consistent so that without the file will work similar to an empty file (the second option above). this also helps balance the shards out of the box on standard projects as generally projects will have longer tests grouped together and shorter tests grouped together. for example when a,b,c,d,e,f,g,h,i, usually it will be something like a,b,c are faster, d,e,f,g are normal, h,i are slower.

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