init: Check umount timeout failure in test

This CL extends the reboot file system integrity test to check for the
UMOUNT_TIMEOUT_MESSAGE(defined in reboot.h) message in pstore logs.

This test can not pass in eng build device until the fix CL: ag/32912227
is landed.

Bug: 407152708
Test: Apply ag/32912227, atest reboot_fs_integrity_test passed
Test: atest reboot_fs_integrity_test failed
Change-Id: I61fb7a00153e08439bad45c3ace9a1895feaa481