ApiSurfaceTest checks the classpath, not the API surface

The change to check the classloader is not wrong - it catches transitive private deps that are nonetheless susceptible to clobber. However, it is checking the wrong classpath. It needs to be an isolated post-shading classpath, not the generic testing classpath.

Likely all the uses are broken.

Imported from Jira [BEAM-3748](https://issues.apache.org/jira/browse/BEAM-3748). Original Jira may contain additional context.
Reported by: kenn.

Start by locating ApiSurfaceTest and all of its uses, then inspect how the generic testing classpath and classloader are assembled. Confirm the intended isolated post-shading classpath and verify that the API-surface checks use it rather than the generic testing classpath.

Written by the indexing model from the issue text.