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
diff --git a/bp2build/Android.bp b/bp2build/Android.bp
index 14e32ed..64ee01f 100644
--- a/bp2build/Android.bp
+++ b/bp2build/Android.bp
@@ -6,6 +6,7 @@
     name: "soong-bp2build",
     pkgPath: "android/soong/bp2build",
     srcs: [
+        "aconfig_conversion_test.go",
         "androidbp_to_build_templates.go",
         "bp2build.go",
         "bp2build_product_config.go",
@@ -21,6 +22,7 @@
     deps: [
         "blueprint-bootstrap",
         "soong-aidl-library",
+        "soong-aconfig",
         "soong-android",
         "soong-android-allowlists",
         "soong-android-soongconfig",