binman: Drop unused return variable for _DoTestFile()

This function returns the exit code from binman, not any data. Fix up a
few callers in the tests.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/ftest.py b/tools/binman/ftest.py
index 75f6ca3..d23967e 100644
--- a/tools/binman/ftest.py
+++ b/tools/binman/ftest.py
@@ -3737,14 +3737,14 @@
     def testEnvironmentNoSize(self):
         """Test that a missing 'size' property is detected"""
         with self.assertRaises(ValueError) as e:
-            data = self._DoTestFile('175_env_no_size.dts')
+            self._DoTestFile('175_env_no_size.dts')
         self.assertIn("'u-boot-env' entry must have a size property",
                       str(e.exception))
 
     def testEnvironmentTooSmall(self):
         """Test handling of an environment that does not fit"""
         with self.assertRaises(ValueError) as e:
-            data = self._DoTestFile('176_env_too_small.dts')
+            self._DoTestFile('176_env_too_small.dts')
 
         # checksum, start byte, environment with \0 terminator, final \0
         need = 4 + 1 + len(ENV_DATA) + 1 + 1