CDRIVER-2053 run tests in only ten threads
CDRIVER-2053 run tests in only ten threads

Previously we had a thread per test; once the number exceeded circa 250,
test-libbson --threads failed on MIPS.

mongodb/libbson,mapr/libbson,ajdavis/libbson,mongodb/libbson,mongodb/libbson,bjori/libbson,mapr/libbson,ajdavis/libbson,rcsanchez97/libbson,rcsanchez97/libbson,bjori/libbson,rcsanchez97/libbson,mapr/libbson,bjori/libbson,ajdavis/libbson