test-encoding: enable fuzz testing of utf8b roundtrip

This test was written by David R. MacIver <[e-mail]> at the London
sprint. We can enable it as the bug in utf8b encoder/decoder has been fixed.

tests/test-encoding.t

@@ -272,3 +272,14 Test roundtrip encoding of lookup tables
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
 
   $ cd ..
+
+Test roundtrip encoding/decoding of utf8b for generated data
+
+#if hypothesis
+
+  >>> from hypothesishelpers import *
+  >>> from mercurial import encoding
+  >>> roundtrips(st.binary(), encoding.fromutf8b, encoding.toutf8b)
+  Round trip OK
+
+#endif