Fix for AV1.TestTell

The tell functions return an unsigned integer.
This causes the AV1.TestTell test case to fail because
-1 is greater than 20 when treated as an unsigned integer.

Change-Id: I9dd1d7eb61260d30d1713a4917159fc6fe8eee42
diff --git a/test/boolcoder_test.cc b/test/boolcoder_test.cc
index c0e6d48..e2ebdda 100644
--- a/test/boolcoder_test.cc
+++ b/test/boolcoder_test.cc
@@ -135,7 +135,7 @@
     }
     const uint32_t expected = (uint32_t)(-kSymbols * log2(probability));
     // Last tell should be close to the expected value.
-    GTEST_ASSERT_LE(last_tell - expected, 20) << " last_tell: " << last_tell;
+    GTEST_ASSERT_LE(last_tell, expected + 20) << " last_tell: " << last_tell;
     // The average frac_diff error should be pretty small.
     GTEST_ASSERT_LE(frac_diff_total / kSymbols, FRAC_DIFF_TOTAL_ERROR)
         << " frac_diff_total: " << frac_diff_total;