test_boottime.py: fixed time extraction regex
The regex used to find the boot time in the log was failing for boot
times lower than 100ms. Fixed it.
@@ -13,7 +13,7 @@ import host_tools.logging as log_tools
MAX_BOOT_TIME_US = 150000
# TODO: Keep a `current` boot time in S3 and validate we don't regress
# Regex for obtaining boot time from some string.
-TIMESTAMP_LOG_REGEX = r'Guest-boot-time\ \=\ (\d+)'
+TIMESTAMP_LOG_REGEX = r'Guest-boot-time\s+\=\s+(\d+)\s+us'
@pytest.mark.timeout(120)
