Disable static_dynamic for cygwin

This test depends on the file command displaying the link status of the
executable.  However, cygwin's files are in PE format - the file command does
not display many details about this format and I wasn't seeing an option that
would enable it to display correctly.  So, skip it here.
Fix prediff in prgenv-* cases for static_dynamic

The cray programming environments link statically by default, as opposed to
our usual environments which default to dynamic.  The prediff for this test
attempted to take that into account, but was checking against the wrong
environment variable to do so.  Fixing that.
The static flag is not supported on darwin, so skip this test there.
Merge pull request #724 from lydia-duncan/fixStaticDynamicTest

Fix static dynamic test [reviewed by David]

This fixes three of the major failing cases for link/sungeun/static_dynamic.

Cygwin: This test depends on the file command displaying the link status of the
executable. However, cygwin's files are in PE format - the file command does
not display many details about this format and I wasn't seeing an option that
would enable it to display correctly. So, skip it here.
PrgEnv-*: The cray programming environments link statically by default, as
opposed to our usual environments which default to dynamic. The prediff for
this test attempted to take that into account, but was checking against the
wrong environment variable to do so. Fixing that.
Darwin: the static flag is not supported on darwin, so running this
test there doesn't really help us.