fix test suite fail in LANG=C

This was caused by 23e9d3bb77

an Arbitrary String is not necessarily encoded using the filesystem
encoding, and in a non-utf8 locale, encodeBS throws an exception on such a
string. All I could think to do is limit test data to ascii.

This shouldn't be a problem in practice, because the all Strings in
git-annex that are not generated by Arbitrary should be loaded in a way
that does apply the filesystem encoding.

Types/MetaData.hs

@@ -270,10 +270,16 @@ instance Arbitrary MetaData where
 		legal k _v = legalField $ fromMetaField k
 
 instance Arbitrary MetaValue where
-	arbitrary = MetaValue <$> arbitrary <*> arbitrary
+	arbitrary = MetaValue 
+		<$> arbitrary
+		-- Avoid non-ascii metavalues because fully arbitrary
+		-- strings may not be encoded using the filesystem
+		-- encoding, which is norally applied to all input.
+		<*> arbitrary `suchThat` all isAscii
 
 instance Arbitrary MetaField where
-	arbitrary = MetaField . CI.mk <$> arbitrary `suchThat` legalField 
+	arbitrary = MetaField . CI.mk 
+		<$> arbitrary `suchThat` legalField 
 
 prop_metadata_sane :: MetaData -> MetaField -> MetaValue -> Bool
 prop_metadata_sane m f v = and