Add subtests to test.Assert

[ivokub]

Added method Run(fn func(assert *test.Assert), descs ...string) to Assert type. This is similar to the Run method in Go's standard testing package to allow running subtests. Refactored tests in places to use subtests.

Removed explicit Fuzz call in integration tests as Fuzz is already called in Assert.ProverSucceeded.

An potential added benefit of using subtests is better coverage with race detector and enabling running subtests in parallel. I didn't implement it now though as Assert uses unsynchronized cache for compiled circuits.

[ivokub]

I went over again over the different range loops and initialized the iteration-local variables everywhere they were used locally.