adb-remount-test: try to fixup device after reboot (fastboot)

If bootloader records a boot failure for a slot, the device can
enter fastboot mode.  If so, set the expected slot and reboot.

Test: adb-remount-test.sh
Bug: 132070014
Change-Id: I801dcec7fd94ff084e54f585370d3c704a0de265