fix: variable modulus subtraction padding

[ivokub]

Description

This PR fixes some of the methods used in testing the variable modulus case in field emulation:

• correctly set the next overflow
• assert that the subtraction padding is big enough
• check that the next overflow wouldn't the native field

We currently only panic when the next overflow would overflow the native field, in contrast to the behaviour with fixed emulated modulus where we then would mod-reduce the value automatically. As modSub is not directly exposed and used in equality assertion (which is then only used in tests), then it should be sufficient for now.

Fixes #1152

Type of change

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

Existing tests pass.

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules