bp2build: Expand check if filegroup contains proto

Previously we looked for proto as suffix, however, some filegroups use
"<foo>-proto-srcs" or "<bar>-proto-sources", instead we look for proto
as a distinct word in a filegroup name.

Test: go test soong tests
Change-Id: Icf916a84304a02617efff9768e5b82d5ffe658bd
diff --git a/bp2build/testing.go b/bp2build/testing.go
index 15cf486..8ae1a38 100644
--- a/bp2build/testing.go
+++ b/bp2build/testing.go
@@ -133,8 +133,8 @@
 		android.FailIfErrored(t, errs)
 	}
 	if actualCount, expectedCount := len(bazelTargets), len(tc.expectedBazelTargets); actualCount != expectedCount {
-		t.Errorf("%s: Expected %d bazel target, got `%d``",
-			tc.description, expectedCount, actualCount)
+		t.Errorf("%s: Expected %d bazel target (%s), got `%d`` (%s)",
+			tc.description, expectedCount, tc.expectedBazelTargets, actualCount, bazelTargets)
 	} else {
 		for i, target := range bazelTargets {
 			if w, g := tc.expectedBazelTargets[i], target.content; w != g {