test/http: avoid huge stack allocations to fix win32 builds Since according to [DOC] default stack size is 1MB, so let's use dynamic allocations instead of changing defaults. DOC: https://msdn.microsoft.com/en-us/library/8cxs58a6.aspx Not-fixes: http/data_length_constraints Fixes: http/lingering_close Fixes: http/non_lingering_close Fixes: https://ci.appveyor.com/project/nmathewson/libevent/build/2.1.5.213