tests: don't spread fixtures.TempDir throughout test cases

tests: don't spread fixtures.TempDir throughout test cases

Instead, provide self.get_temp_file_path() utility method for tests
interested in creating temporary files.

There also cases when tests are interested in multiple separate
temporary directories. With this in mind, self.get_temp_file_path()
supports root= argument that allows to pass a different temporary
directory fixture than default.

While at it, consolidated cleanup setup for NEC temporary file in single
place.

Change-Id: Ie041edcfde1b16183244a3e6068658308d2a67f5