Rename assertNotNil to assertNoError, since the former name is incorrect

[ethan-lowman-dd]

The assertNotNil helper functions were actually asserting that the error was nil. This PR renames the functions to reflect their actual behavior.

[coveralls]

Pull Request Test Coverage Report for Build 1931133902

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 70.23%

---

Totals
Change from base Build 1924706088:	0.0%
Covered Lines:	2168
Relevant Lines:	3087

---

💛 - Coveralls

[joshuagl]

LGTM, thanks @ethan-lowman-dd.

I assume we're carrying four copies of the same function to avoid the overhead of defining some kind of test utils module (package?)?

[ethan-lowman-dd]

@joshuagl I assume so. I'd rather not add another testing module, since the codebase already uses at least two (https://pkg.go.dev/github.com/stretchr/testify and https://pkg.go.dev/gopkg.in/check.v1)