Move shared test code from ext/ to tests/

[c24t]

opentelemetry-ext-testutil is a package with shared test classes used by ext packages (right now only opentelemetry-ext-flask). We don't release this package, just import it in other tests.

Right now, on each release, we build everything in ext/. This means whoever does the release has to remember to exclude this package when they push the others to PyPI.

This PR moves the files and package:

• Move files ext/opentelemetry-ext-testutil -> tests/util
• Move package opentelemetry.ext.testutil -> opentelemetry.test

This makes maintainers' lives easier, but it does mean that other packages that use testutils will have to install install the opentelemetry.test package from source. But this is already the case since we don't publish opentelemetry-ext-testutil.

This PR proposes that we move shared test code back into the main repo until we move a package that depends on it into a separate repo, at which point we'll have to put this code in its own top-level package.

[mauriciovasquezbernal]

opentelemetry-ext-testutil is a package with shared test classes used by ext packages (right now only opentelemetry-ext-flask).

For the record, it's also used in opentelemetry-ext-wsgi

This PR proposes that we move shared test code back into the main repo until we move a package that depends on it into a separate repo, at which point we'll have to put this code in its own top-level package.

I think it could be better to move it to its own package right now. We are already starting to push on https://github.com/open-telemetry/opentelemetry-auto-instr-python so it could be useful there.

[codeboten]

This makes sense to me. I would suggest that publishing a test package would make this useful for anyone writing exporters or integrations, so it might make sense to publish it sooner than later. Though i guess anyone wanting to use this can already do so via source so it's not that urgent