Clean up and split off the GMP test suite

[treeswift]

lxe@WALNUC:~/Code/mxe-shared$ ls -l `find -name libtests.la`
lrwxrwxrwx 1 lxe lxe  14 Feb  1 00:00 ./usr/armv7-w64-mingw32/bin/gmp-tests/.libs/libtests.la -> ../libtests.la
-rw-r--r-- 1 lxe lxe 925 Feb  1 00:00 ./usr/armv7-w64-mingw32/bin/gmp-tests/libtests.la
lxe@WALNUC:~/Code/mxe-shared$ find usr/armv7-w64-mingw32/bin -name '*.c' | head
usr/armv7-w64-mingw32/bin/gmp-tests/x86check.c
usr/armv7-w64-mingw32/bin/gmp-tests/t-modlinv.c
usr/armv7-w64-mingw32/bin/gmp-tests/mpn/t-asmtype.c
usr/armv7-w64-mingw32/bin/gmp-tests/mpn/t-toom62.c
usr/armv7-w64-mingw32/bin/gmp-tests/mpn/t-hgcd_appr.c
usr/armv7-w64-mingw32/bin/gmp-tests/mpn/t-toom6h.c
usr/armv7-w64-mingw32/bin/gmp-tests/mpn/logic.c
usr/armv7-w64-mingw32/bin/gmp-tests/mpn/t-invert.c
usr/armv7-w64-mingw32/bin/gmp-tests/mpn/t-toom6-sqr.c
usr/armv7-w64-mingw32/bin/gmp-tests/mpn/t-toom52.c
lxe@WALNUC:~/Code/mxe-shared$

We probably want gmp-tests to be a separate deployment unit, and libtool metadata not to be published at all.