tests/functional: Allow running TCG plugins tests on non-Linux/BSD hosts

tests/functional: Allow running TCG plugins tests on non-Linux/BSD hosts

Not all platforms use the '.so' suffix for shared libraries,
which is how plugins are built. Use the recently introduced
dso_suffix() helper to get the proper host suffix.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2804
Suggested-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20250220080215.49165-4-philmd@linaro.org>
[AJB: moved plugin_file into testcase.py]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250304222439.2035603-11-alex.bennee@linaro.org>