ECH specific padding doc, test and tweak #985

Job	Run time
run-checker (enable-asan enable-ubsan no-shared no-asm -DOPENSSL_SMALL_FOOTPRINT)	10m 36s
run-checker (no-ct)	5m 18s
run-checker (no-dso)	5m 4s
run-checker (no-dynamic-engine)	5m 8s
run-checker (no-ec2m)	5m 4s
run-checker (no-engine no-shared)	4m 19s
run-checker (no-err)	5m 27s
run-checker (no-filenames)	5m 19s
run-checker (enable-ubsan no-asm -DOPENSSL_SMALL_FOOTPRINT -fno-sanitize=alignment)	8m 17s
run-checker (no-module)	5m 1s
run-checker (no-ocsp)	5m 14s
run-checker (no-pinshared)	5m 23s
run-checker (no-srp)	5m 16s
run-checker (no-srtp)	5m 23s
run-checker (no-ts)	5m 16s
run-checker (no-integrity-only-ciphers)	5m 31s
run-checker (enable-weak-ssl-ciphers)	5m 18s
run-checker (enable-zlib)	5m 22s
run-checker (enable-pie)	5m 16s
	1h 47m 32s

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