tests/kms_vrr: Add a subtest for seamless modeset to a virtual LRR mode

This subtest validates LRR fastset functionality by testing seamless switching
without full modeset to any of the virtual LRR mode within VRR range.
Here we start from highest refresh rate mode and then switch to virtual LRR
modes in the steps of 10Hz within the VRR range.

This is used to test the LRR fastset functionality of the driver.

v5:
- Pass a reference to flip_and_measure (bhanu)
- Add a new line between between declaration and code logic

v4:
- Change the test name to align with drrs/vrr tests (Bhanu)
- Fix some build warnings due to rebase
- Use a local virtual_mode variable

v3:
- Fix build error due to rebase (Manasi)

Cc: Drew Davenport <ddavenport@chromium.org>
Cc: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Manasi Navare <navaremanasi@chromium.org>
Reviewed-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>