TST: split out melt-tests in pandas/tests/reshape/test_reshape.py

[jreback]

since melt and friends were moved to pandas/core/reshape/melt.py want to follow suit with the tests.

if you want to go once further and split out the melt, wide_to_long and lreshape tests to a new sub-module, e.g. pandas.tests.reshape.melt.test_melt, test_lreshape, test_wide_to_long would not object.

[jreback]

cc @tdpetrou

[tdpetrou]

Sure I can do this. I'll just keep it to one file test_melt for now.