Use new testing packages in x/tools for integration tests

[zombiezen]

golang.org/x/tools now includes golang.org/x/tools/go/packages/packagestest — test infrastructure to set up Go workspace layouts. Using this infrastructure would allow us to remove our bespoke code to accomplish the same goal.

Further, x/tools now also includes golang.org/x/tools/go/expect, a convenient way of being able to use source code position information inside tests in a stable fashion. We should leverage this to start verifying error message positions.