ngtests: improve _assert* methods

[sechkova]

Description of issue or feature request:

Currently #1636 proposes three types of asserts for checking the state of local metadata:

• _assert_files_exist
• _assert_content_equals
• _assert_version_equals

These still have some issues (see https://github.com/theupdateframework/python-tuf/pull/1636#discussion_r74079588, https://github.com/theupdateframework/python-tuf/pull/1636#pullrequestreview-794981714)

• non-trivial to pick which assert to use in which test case
• _assert_content_equals calls the private method Repositorysimulator._fetch_metadata() to get the metadata content in bytes.

A smarter approach is needed, be it integrated in Repositorysimulator or not.