Move unit tests to the right place

[ales-otf]

Description

This PR moves the tests from pallet-subtensor and pallet-admin-utils to the place, where Rust will see them as unit tests. That means, to make conditional compilation (cfg(test)) as well as access to private API and data fields in tests available. Currently these tests are located so, that Rust sees them as integration tests, which forces us to add public APIs needed to test private API.

Basically, the changes are very simple:

• pallets/subtensor/tests moved to pallets/subtensor/src/tests
• pallets/admin-utils/tests moved to pallets/admin-utils/src/tests

This PR will allow further codebase clean ups. Particularly, removing the public API, which exists only for testing.

Related Issue(s)

• Closes Consider unit tests preference over integration tests #926

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run cargo fmt and cargo clippy to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[gztensor]

I like this change: Now tests have access to private functions and cfg(test) also works.