Fix mysql-test/lib/mtr_cases.pm to search tests in submodules

This task is a dependency for MCOL-4674.
It's not possible to run ColumnStore MTR tests using:

cd mysql-test

./mtr columnstore/basic

Two problems are reported in MCOL-4674 and should be fixed inside ColumnStore repository.

The third problem should be fixed in the server repository:

Currently mysql-test/lib/mtr_cases.pm searches tests in:

storage/XXX/mysql-test/XXX

and therefore supports only these cases:

• both plugin sources and MTR tests for this plugin are in the server repository (e.g. storage/connect)
• the plugin sources are inside a submodule, but the plugin tests are in the server repository (e.g. storage/rocksdb)

but it does not support the case when:

• both plugin sources and MTR tests for this plugin reside in a submodule (e.g. storage/columnstore)

We should fix mtr_cases.pm to additionally search for tests in:

storage/XXX/XXX/mysql-test/XXX

This fix for mtr_cases.pm does the job:

diff --git a/mysql-test/lib/mtr_cases.pm b/mysql-test/lib/mtr_cases.pm

index b61333535dc..dd003dea31d 100644

--- a/mysql-test/lib/mtr_cases.pm

+++ b/mysql-test/lib/mtr_cases.pm

@@ -64,8 +64,8 @@ my $plugin_suitedir_regex;

 my $overlay_regex;

 if (-d '../sql') {

-  @plugin_suitedirs= ('storage/*/mysql-test', 'plugin/*/mysql-test');

-  $overlay_regex= '\b(?:storage|plugin)/(\w+)/mysql-test\b';

+  @plugin_suitedirs= ('storage/*/mysql-test', 'plugin/*/mysql-test', 'storage/*/*/mysql-test', );

+  $overlay_regex= '\b(?:storage|plugin|storage[/][^/]*)/(\w+)/mysql-test\b';

 } else {

   @plugin_suitedirs= ('mysql-test/plugin/*');

   $overlay_regex= '\bmysql-test/plugin/(\w+)\b';