Reduce the size of Big String and Big Binary tests to 2**14 (minus one

for Big String).  This should make the tests pass on Win98SE.  Note
that the docs only promise lengths up to 2048.  Unfortunately this no
longer tests for the segfault I was seeing earlier, but I'm confident
I've nailed that one. :-)  Fixes SF 852281.  Will backport to 2.3.