tests/avocado: Rename avocado_qemu.Test -> QemuSystemTest

To run user-mode emulation tests, we introduced the
avocado_qemu.QemuUserTest which inherits from avocado_qemu.QemuBaseTest.
System-mode emulation tests are based on the avocado_qemu.Test class,
which also inherits avocado_qemu.QemuBaseTest. To avoid confusion,
rename it as avocado_qemu.QemuSystemTest.

Suggested-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211105143416.148332-7-f4bug@amsat.org>