bootstat: test: check ro.boot.bootreason in some tests

blind_reboot_test() did not report the ro.boot.bootreason value,
making it more difficult to diagnose failures.

Test: system/core/bootstat/boot_reason_test.sh cold warm hard
Bug: 63736262
Change-Id: I313cfef202b1e06c583b0b47cd5d0e0888a7dbe7