Make pprof driver tests less noisy. (#349)

internal/driver/fetch_test.go

 			}
 			f.args = tc.sources
 
-			o := setDefaults(nil)
-			o.Flagset = f
+			o := setDefaults(&plugin.Options{
+				UI:      &proftest.TestUI{T: t, AllowRx: "Local symbolization failed|Some binary filenames not available"},
+				Flagset: f,
+			})
 			src, _, err := parseFlags(o)
 
 			if err != nil {

internal/proftest/proftest.go

 	// implementation does. Without this Error() calls fmt.Sprintln() which
 	// _always_ adds spaces between arguments, unlike fmt.Sprint() which only
 	// adds them between arguments if neither is string.
-	ui.T.Error(fmt.Sprint(args...))
+	ui.T.Error("unexpected error: " + fmt.Sprint(args...))
 }
 
 // IsTerminal indicates if the UI is an interactive terminal.