template(tests): replace `assert` with `require` from `testify` packages

[lumtis]

Is your feature request related to a problem? Please describe.
In test templates, we use require from testify most of the time and assert in some places.
I think it would be better for code consistency to use require everywhere, except if the use of assert eventually makes sense.

Describe the solution you'd like
Replace assert everywhere

I don't put it in #1409 because this issue is eventually open for discussion

[ilgooz]

I think require is the safest option. I would apply the same for starport's own source code.

[Pantani]

+1 require