testsuite: Fix test suite prepare guide

Remove deprecated yaml-to-mux plugin and yml variants from the doc.
Also remove subprocess32 backport installation which is not used.

Signed-off-by: Anton Mikanovich <amikan@ilbers.de>

testsuite/README.md

@@ -4,29 +4,14 @@ The framework could be installed by using standard HOWTO:
 
   https://github.com/avocado-framework/avocado#installing-with-standard-python-tools
 
-Then you need to install varianter yaml-to-mux plugin by following these instructions:
-
-  https://github.com/avocado-framework/avocado/tree/master/optional_plugins
-
-## For Debian 9.x
-
-        $ sudo apt-get install python-pip
-        $ pip install --user subprocess32
-        $ pip install --user avocado-framework
-        $ pip install --user avocado-framework-plugin-varianter-yaml-to-mux
-
-# Pre
-
-        $ export PATH=$PATH:~/.local/bin
-        $ cd isar
-        $ source isar-init-build-env
+## For Debian (tested on Debian 10.x)
 
+        $ sudo dpkg -i avocado_91.0_all.deb
 
 # Run test
 
 Each testsuite directory contains:
- - run.sh - script to start tests
- - variants.yaml - set of input data
+ - run_*.sh - script to start tests
  - *.py - test case
 
 # Other