Migrate testify/assert and testify/require dependencies to use gotest.tools

[tshakalekholoane]

There are some limitations with stretchr/testify i.e. issues #535, #187 that make it unsuitable for use in some cases. These do not apply to gotest.tools/assert and as a result a decision has been taken to migrate all assertion helpers to this package.

There is also a command line tool that could be of use.