Update test plan Modeselect 1.2 at Step 2 to confirm supported mode values are unique.

[jtrejoespinoza-grid]

Summary Title:

Update the test plan at: https://github.com/CHIP-Specifications/chip-test-plans/blob/master/src/cluster/modeselect.adoc#tc-mod-1-2-cluster-attributes-with-server-as-dut to ensure the mode numbers are unique.

Description:

Based the comments at: project-chip/connectedhomeip#37476
Update the test plan on mode select to verify supported modes numbers are unique.
In the step2 from the test case 1.2 add the following:

Verify that each supported mode number is unique.