Improvement: Remove testify from production binaries

**Is your feature request related to a problem? Please describe.**

The `vault` production binary currently includes the `github.com/stretchr/testify` library.

The `github.com/stretchr/testify/assert` package is being imported within production source files rather than being restricted to `_test.go` files. Because `testify` includes global variable initializers (e.g. [here](https://github.com/stretchr/testify/blob/master/assert/assertion_compare.go)), Go cannot eliminate the code, even if the functions themselves are likely unreachable in production.

**Describe the solution you'd like**

I would like `testify` to be completely eliminated from the production binary.

Motivation:

* It's a bit strange for a tool like Vault to include this unrelated test package in the final build.
* It might cause extra work if there's a vulnerability anywhere in the `testify` project, or its dependencies, it will show up as a vulnerability for Vault too

**Describe alternatives you've considered**

**Explain any additional use-cases**

**Additional context**

Here is minimal "reproduction" case on how the additional dependency is introduced with a trivial Go program:

```
$ go mod init example.com/test
$ go get github.com/stretchr/testify
$ cat > main.go <

Start by locating production, non-_test.go imports of github.com/stretchr/testify/assert and inspect how they enter the Vault build. Use go build and go tool nm to compare dependencies and symbols; done means the production binary no longer includes testify while the existing tests still pass.

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