test: rename 'shared' directory to testdata

What does this MR do?

rename 'shared' directory to testdata.
testdata is the official/idiomatic way to store additional test files.

See https://github.com/golang/go/issues/14715 and https://github.com/golang/go/commit/2f9eeea212681927385a4713004438cd864966e0

TODO

• I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines