Initial test for dt for early mount.

Basic tests for some of the requirements stipulated by
https://source.android.com/devices/architecture/kernel/modular-kernels#mounting-partitions-early-first-stage-mount
namely:

1. All early mount fstab entries have required properties present.
2. Compatible string is set to android,fstab.

Bug: 72505967
Test: build, run this test.

Change-Id: Icde5e49a7526fa23c1df52cc51f8f8ace7ddff6e