binman: Avoid unwanted output in testFitFirmwareLoadables()

This prints a message about the missing tee-os generated by the test. This
is confusing, so suppress it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/ftest.py b/tools/binman/ftest.py
index 0b3bca9..d5cae38 100644
--- a/tools/binman/ftest.py
+++ b/tools/binman/ftest.py
@@ -6353,10 +6353,11 @@
             'tee-os-path': 'missing.bin',
         }
         test_subdir = os.path.join(self._indir, TEST_FDT_SUBDIR)
-        data = self._DoReadFileDtb(
-            '276_fit_firmware_loadables.dts',
-            entry_args=entry_args,
-            extra_indirs=[test_subdir])[0]
+        with test_util.capture_sys_output() as (stdout, stderr):
+            data = self._DoReadFileDtb(
+                '276_fit_firmware_loadables.dts',
+                entry_args=entry_args,
+                extra_indirs=[test_subdir])[0]
 
         dtb = fdt.Fdt.FromData(data)
         dtb.Scan()