test: make blob desiredSize assertion robust #8398

Job	Run time
Build slim tarball	1m 22s
aarch64-linux: Cache V8 build	25s
aarch64-darwin: with shared libraries	25m 2s
x86_64-linux: with shared libraries	26m 46s
x86_64-darwin: with shared libraries	40m 35s
aarch64-linux: with shared openssl-3.5.7	23m 26s
aarch64-linux: with shared openssl-3.6.2	23m 11s
aarch64-linux: with shared openssl-4.0.1	22m 49s
aarch64-linux: with shared openssl-1.1.1w	23m 11s
aarch64-linux: with shared openssl-3.0.21	22m 59s
aarch64-linux: with shared boringssl-0.20260526.0	22m 56s
	3h 52m 42s

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