Fix soong for go test ./...

Fix various issues that prevent building or running tests with the
Go tools:

Remove testing/test_spec_proto/go.mod.  There's already a go.mod at
the top level, and adding testing/test_spec_proto/go.mod breaks
compiling with the Go tools in IntelliJ or from the command line.

Fix go vet issues when trying to print []DataPath.

Fix aconfig_conversion_test.go and add it to Android.bp so it runs
in presubmit.

Test: go test ./...
Change-Id: I18af2f05631446fe507744a2951e4b02c7ab59be