Standardized testdata config filepath pattern

[rmfitzpatrick]

Is your feature request related to a problem? Please describe.
As mentioned in #6459 (comment), there is a current test pattern for specifying config files of the form:

cfg, err := configtest.LoadConfigAndValidate(path.Join(".", "testdata", "config.yaml"), factories)

This invocation is slightly problematic for two reasons:

1. the initial "." is unnecessary.
2. path.Join() hardcodes the separator as "/" instead of picking it up from the os like filepath.Join() does, which is used by the underlying koanf loader.

Describe the solution you'd like
Replace all path.Join(".", with filepath.Join(

Describe alternatives you've considered
Using embed could be preferable to using paths and loaders overall since the underlying byte slice would be available in the packages. I have superficially confirmed that embedded items in test functions don't make their way to the final build using hexdump, but a more rigorous assessment would be necessary. Since the existing helper works with paths and uses* the config map loaders, requiring embed directives as replacements and using bytes directly is probably overkill for most cases.

[jpkrohling]

Your proposed solution seems simple and effective enough.

[DiptoChakrabarty]

@jpkrohling @rmfitzpatrick I would like to work on this.

[jpkrohling]

Go for it, @DiptoChakrabarty

[bogdandrutu]

@jpkrohling can you track to ensure we do the same in the core repo?

[jpkrohling]

@codeboten beat me to it, thanks!