docs: Extend development tricks with dynamic test override

docs/WritingTests.asciidoc

@@ -1134,11 +1134,14 @@ stop them outputting non-printable characters.
 
 
 == Test Development tricks
-=== Modifying setting of an existing test
+=== Trigger new tests by modifying settings from existing test runs
 
-There is no interface to modify existing tests but the clone_job.pl script
-can be used to create a new job that adds, removes or changes
-settings. This script is located at +/usr/share/openqa/script/+.
+To trigger new tests with custom settings the command line client
++openqa-client+ can be used. To trigger new tests relying on all settings from
+existing tests runs but modifying specific settings the +openqa-clone-job+
+script can be used. Within the openQA repository the script is called
++clone_job.pl+ and is located at +/usr/share/openqa/script/+.  This tool can
+be used to create a new job that adds, removes or changes settings.
 
 [source,sh]
 --------------------------------------------------------------------------------
@@ -1163,7 +1166,7 @@ and the job will not appear as a job in any job group, e.g.:
 clone_job.pl --from localhost 42 _GROUP=0
 -------------
 
-==== Backend variables for faster test execution
+=== Backend variables for faster test execution
 
 The +os-autoinst+ backend offers multiple test variables which are helpful for
 test development. For example:
@@ -1248,3 +1251,47 @@ the clone source job or specifying the variable explicitly:
 ----
 clone_job.pl --from https://openqa.opensuse.org  --host localhost 24 TESTDEBUG=1 SKIPTO=consoletest-yast2_i
 ----
+
+=== Defining a custom test schedule or custom test modules
+
+Normally the test schedule, that is which test modules should be executed and
+which order, is prescribed by the +main.pm+ file within the test distribution.
+Additionally it is possible to exclude certain test modules from execution
+using the os-autoinst test variables +INCLUDE_MODULES+ and +EXCLUDE_MODULES+ as
+well as define a custom schedule using the test variable +SCHEDULE+.
+Also test modules can be defined and overridden on-the-fly using a
+downloadable asset.
+
+For example one can schedule a job on a production instance with a custom
+schedule consisting of two modules from the provided test distribution plus
+one test module which is defined dynamically and downloaded as an asset from
+an external trusted download domain:
+
+----
+openqa-clone-job --from https://openqa.opensuse.org --host https://openqa.opensuse.org 24 SCHEDULE=tests/boot/boot_to_desktop,tests/console/consoletest_setup,foo ASSET_1_URL=https://example.org/my/test/foo.pm
+----
+
+where "foo.pm" is a file in test module format, e.g. the following content:
+
+----
+use base 'consoletest';
+use strict;
+use testapi;
+
+sub run {
+    select_console 'root-console';
+    assert_script_run 'foo';
+}
+
+sub post_run_hook {}
+1;
+----
+
+For example this can be used in bug investigations or trying out new test
+modules which are hard to test locally.
+https://github.com/os-autoinst/os-autoinst/blob/master/doc/backend_vars.asciidoc
+describes the +SCHEDULE+ parameter in details as well as the others. The
+section "Asset handling" in the <<UsersGuide.asciidoc#usersguide,Users Guide>>
+describes how downloadable assets can be specified. It is important to note
+that the specified asset is only downloaded once. New versions must be
+supplied as new, unambiguous download target file names.