test: mark stringbytes-external-exceed-max tests as flaky on AIX #624

Job	Run time
Build slim tarball	1m 31s
aarch64-darwin: with shared libraries	40m 47s
x86_64-darwin: with shared libraries	41m 24s
aarch64-linux: with shared libraries	42m 43s
x86_64-linux: with shared libraries	49m 3s
	2h 55m 28s

Provide feedback