uasc: return an error for invalid uri/mode combinations with None

[magiconair]

The previous code forced security mode None if the URI was None changing
the incoming Config object. While it makes the code more robust it also
has a side effect and we were already checking for other invalid
combinations. This patch puts all checks in one place and returns an
error if the combination is invalid.

[magiconair]

I've added https://github.com/stretchr/testify as a dependency for testing. I'm open for using something else like https://github.com/gotestyourself/gotest.tools if there are strong opinions. What do others think?

Update: just in the same file we already use the verify package and if err != nil { t.Fatal(err) } pattern. Adding require would add another pattern for testing the same things which probably adds more confusion than it is worth.

I've implemented the errorContains function which I've needed for the test and removed the stretchr dependency again.

[kung-foo]

FWIW, I really like the testify (require) package and we use it in 100% of our internal go projects.

[kung-foo]

LGTM

[magiconair]

We use testify as well. For this one I was more concerned with having three different testing strategies in the codebase. Didn't seem worth the effort.