Re-add backtrace note about unreadable elf.

When moving to the proto-ized tombstones, the note about unreadable
elf files in a backtrace got lost. This re-adds it and adds a test
to verify that the note properly shows up.

Bug: 185428454

Test: Ran unit tests.
Change-Id: I1150cc737772e1b79fd73ec5c782caadc4629421
diff --git a/debuggerd/Android.bp b/debuggerd/Android.bp
index 7b6f6c0..198e4de 100644
--- a/debuggerd/Android.bp
+++ b/debuggerd/Android.bp
@@ -276,6 +276,12 @@
     ],
 }
 
+cc_test_library {
+    name: "libcrash_test",
+    defaults: ["debuggerd_defaults"],
+    srcs: ["crash_test.cpp"],
+}
+
 cc_test {
     name: "debuggerd_test",
     defaults: ["debuggerd_defaults"],
@@ -341,6 +347,10 @@
         },
     },
 
+    data: [
+        ":libcrash_test",
+    ],
+
     test_suites: ["device-tests"],
 }