Add test for current sort import.

This add some test for what was requested in #13 and implemented in #263

tests/tests_sorts.py

@@ -0,0 +1,54 @@
+from   pyflyby._parse     import PythonBlock
+from   pyflyby._imports2s import sort_imports
+from textwrap import dedent
+import pytest
+
+code1 = dedent("""
+        import external
+        import os
+        import sympy
+        import numpy
+        import json
+        import simplejson
+
+        from pkg1.mod1 import foo
+        from pkg1.mod2 import bar
+        from pkg2 import baz
+        import yy
+
+        from pkg1.mod1 import foo2
+        from pkg1.mod3 import quux
+        from pkg2 import baar
+        import zz
+    """)
+
+# The logic requested in issue 13 whas to keep blocks, but order
+# lexicographically. we do seem to be splitting stdlib from installed packages,
+# but not adding a blank line and then sort lexicographically if an package has
+# several submodules importted we put it in a block, otherwise we keep
+# everything together.
+
+expected1 = dedent("""
+        import json
+        import os
+        import external
+        import numpy
+
+        from pkg1.mod1 import foo, foo2
+        from pkg1.mod2 import bar
+        from pkg1.mod3 import quux
+
+        from pkg2 import baar, baz
+        import simplejson
+        import sympy
+        import yy
+        import zz
+    """)
+
+@pytest.mark.parametrize('code, expected',[(code1, expected1)])
+def test_sort_1(code, expected):
+
+    assert str(sort_imports(PythonBlock(code))) == expected
+    # expected is stable
+    assert str(sort_imports(PythonBlock(expected))) == expected
+