Add test to ReadFstabFromDt in the VtsGsiBootTest

This test is added to address the init behavior in
first_stage_mount in order to make sure /metadata can be early
mounted on devices that still use dt-fstab.

Bug: 138449350
Test: vts-tradefed> run vts -m VtsGsiBootTest
Change-Id: If1c5f037f7850f6237e60f3e0cb19f429a3abebd
1 file changed