bootutil: mynewt: Fix boot_serial unit tests #3989

Job	Run time
environment (esp32s2, serialrecovery)	1m 47s
environment (esp32h2, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	2m 0s
environment (esp32, serialrecovery)	1m 45s
environment (esp32c2, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	2m 1s
environment (esp32c6, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	2m 0s
environment (esp32c3, serialrecovery)	1m 40s
environment (esp32c2, serialrecovery)	1m 49s
environment (esp32s2, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	2m 6s
environment (esp32, multi-image,multi-boot, multi)	1m 44s
environment (esp32s3, serialrecovery)	2m 5s
environment (esp32h2, serialrecovery)	1m 47s
environment (esp32, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secureb...	2m 2s
environment (esp32c3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	2m 3s
environment (esp32s3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	2m 1s
environment (esp32c6, serialrecovery)	1m 45s
environment (esp32s3, multi-image,multi-boot, multi)	2m 2s
	30m 37s

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