(PUP-3014) Accept. tests - 3.7 PMT updates generate

This commit adds additional acceptance test coverage for updates
to the puppet module generate command. These tests validate the
following:

* (PUP-2781) Generated requirements should use the version_requirement field
* (PUP-2691) Generate should succeed when skeletons use the 'description'
  metadata property.
* (PUP-2079) Generate should support erb skeleton templates"
(PUP-3014) Skip erb skeleton tests on Windows

This commit skips erb skeleton tests on Windows.

The module tool does not currently honor skeleton files for
privileged users. Running these tests as a non-privileged user
on Windows does not work because cygwin does not support 'su'.
(PUP-3014) PMT test basic_generate teardown fix

This commit moves the clean up step in the PMT
generate/basic_generate test into a teardown step in order to
ensure that the files are removed before other tests run.