adb-remount-test: Reduce noise from initial overlayfs diagnostics

There's quite a lot of noise from running "Checking current overlayfs
status". Improve the test output by filtering uninteresting df lines.

* "/apex/..." mounts not interesting.
* "rw" mounts not interesting.
* "fuse" devices not interesting.

Bug: 243116800
Test: adb-remount-test
Change-Id: Id15844d853aaf3f7ed86f1a83544494b697b5b39