VtsTrebleVintfTest: HalsAreServed print instance

The instance name from this error message was missing which made it
harder to pin down b/117953904.

Fixes: 117957368
Test: VtsTrebleVintfTest
Change-Id: Ife4d3a2823dc72a71de65037315a5f83096c91bc
diff --git a/treble/vintf/SingleManifestTest.cpp b/treble/vintf/SingleManifestTest.cpp
index ecd5a81..42acfc2 100644
--- a/treble/vintf/SingleManifestTest.cpp
+++ b/treble/vintf/SingleManifestTest.cpp
@@ -147,8 +147,8 @@
       Partition partition = GetPartition(hal_service);
       if (partition == Partition::UNKNOWN) return;
       EXPECT_EQ(expected_partition, partition)
-          << fq_name.string() << " is in partition " << partition
-          << " but is expected to be in " << expected_partition;
+          << fq_name.string() << "/" << instance_name << " is in partition "
+          << partition << " but is expected to be in " << expected_partition;
     };
   };