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>

tools/binman/ftest.py

@@ -6353,6 +6353,7 @@ fdt         fdtmap                Extract the devicetree blob from the fdtmap
             'tee-os-path': 'missing.bin',
         }
         test_subdir = os.path.join(self._indir, TEST_FDT_SUBDIR)
+        with test_util.capture_sys_output() as (stdout, stderr):
             data = self._DoReadFileDtb(
                 '276_fit_firmware_loadables.dts',
                 entry_args=entry_args,