[2.0] Document the changed model of providing options to test_package

As per https://github.com/conan-io/conan/issues/13241, options on the package-under-test can no longer be set in `configure()` or passed as an option to `requires()` or `test_requires()` (as per #3026). They need to be passed via the command line or profile for consistency of the interface.

This would be useful to document so that people understand how they can test a variety of package configurations effectively. The previous method might have been to write a number of test_package's, however now it can be done just by varying profile/cli invocation.

See details in #13241 for more details.

Read issues #13241 and #3026 first to understand the changed test_package options behavior. Locate the documentation section covering test_package configuration, then explain how profiles or command-line invocation vary package configurations and clarify that options are no longer set through configure(), requires(), or test_requires().

Written by the indexing model from the issue text.