gmp, test: fix all kinds of require/load brain damage.

Tests were still run by the runtests() function using load instead
of include. Combined with the fact that the LOAD_PATH includes the
current directory and that there are both test files and source
files named "bigint.jl" and "bigfloat.jl" was causing all sorts of
mistaken file loading hilarity. This mess took quite some time to
unravel: using cd(()->require("gmp"),"..") was avoiding the issue
because by cd'ing into the top-level julia directory, the test dir
was no longer in LOAD_PATH (as the current directory).