Add a diff test phony target per partition

This enables running diff test per partition indenpendently.

Test: m soong_generated_system_filesystem_test
Change-Id: Ie897b7c5bcb66a676c7fbbdb48207082213eac77
diff --git a/fsgen/filesystem_creator.go b/fsgen/filesystem_creator.go
index f446c2b..3cee7e4 100644
--- a/fsgen/filesystem_creator.go
+++ b/fsgen/filesystem_creator.go
@@ -499,10 +499,14 @@
 
 	var diffTestFiles []android.Path
 	for _, partitionType := range f.properties.Generated_partition_types {
-		diffTestFiles = append(diffTestFiles, f.createDiffTest(ctx, partitionType))
+		diffTestFile := f.createDiffTest(ctx, partitionType)
+		diffTestFiles = append(diffTestFiles, diffTestFile)
+		ctx.Phony(fmt.Sprintf("soong_generated_%s_filesystem_test", partitionType), diffTestFile)
 	}
 	for _, partitionType := range f.properties.Unsupported_partition_types {
-		diffTestFiles = append(diffTestFiles, createFailingCommand(ctx, fmt.Sprintf("Couldn't build %s partition", partitionType)))
+		diffTestFile := createFailingCommand(ctx, fmt.Sprintf("Couldn't build %s partition", partitionType))
+		diffTestFiles = append(diffTestFiles, diffTestFile)
+		ctx.Phony(fmt.Sprintf("soong_generated_%s_filesystem_test", partitionType), diffTestFile)
 	}
 	ctx.Phony("soong_generated_filesystem_tests", diffTestFiles...)
 }