tests

Add download speed limit configuration option. #401

Job	Run time
docs	32s
uncrustify	38s
build (-Dsystemd=enabled)	4m 21s
build (-Dsystemd=enabled, CFLAGS="-fsanitize=address -fsanitize=leak -g" LDFLAGS="-fsanitize=addr...	4m 23s
build (-Dsystemd=disabled, CFLAGS="-fsanitize=address -fsanitize=leak -g" LDFLAGS="-fsanitize=add...	4m 29s
build (-Dsystemd=disabled)	4m 34s
	18m 57s

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