fix #26: validation of decimal facets

An inappropriate tearDown that changed the decimal context to a
non-default value in an unrelated test caused test-trac-0091 to truncate
results, resulting in failure to discover a too-precise result, but only
when run in the suite that first ran test-decimal.

Likely this only ever worked because in other versions test-trac-0091
was run first.

tests/datatypes/test-decimal.py

@@ -11,9 +11,6 @@
 from pyxb.exceptions_ import *
 
 class Test_decimal (unittest.TestCase):
-    def tearDown (self):
-        decimal.setcontext(decimal.BasicContext)
-
     def assertAlmostEqual (self, v1, v2, *args, **kw):
         if (isinstance(v1, Decimal)
             or isinstance(v2, Decimal)):