Ignore all MSVS projects generated when running tests.

Previously project files were ignored everywhere in the tree, which was too
broad, and at the same time pre-MSBuild files with .vcproj extension were not
ignored where they were created by the tests.

Do ignore them, and .vcxproj MSBuild files too, but only under tests/projects
where they are expected to be created.

Also move *Makefile* from the main .gitignore to under tests/projects for
consistency and ignore Makefile.suncc created during hello_world test too.
Fix wrong comment in one of the test_model tests.

The comment was copy-and-pasted from another test.
Fix too eager expansion of variables inside "sources" statement.

Keep variables in the names of the sources unexpanded, otherwise they could be
evaluated too early, e.g. in the context of the base template where variables
had different values from their real values in the most derived target.

This was inconsistent with the use of variables in all the other contexts and
not very useful as it didn't allow the final target to specify where its
sources should be found, so fix this by not performing this early expansion.