fine tune how to split the test suite

[gnzlbg]

The test suite was split in #62 into three test groups:

RUSTFLAGS="${ORIGINAL_RUSTFLAGS} --cfg test_v16  --cfg test_v32" cargo_test ${1}
RUSTFLAGS="${ORIGINAL_RUSTFLAGS} --cfg test_v64  --cfg test_v128" cargo_test ${1}
RUSTFLAGS="${ORIGINAL_RUSTFLAGS} --cfg test_v256 --cfg test_v512" cargo_test ${1}

We should fine tune this, and see if a different split, for example:

RUSTFLAGS="${ORIGINAL_RUSTFLAGS} --cfg test_v16  --cfg test_v32 --cfg test_v64" cargo_test ${1}
RUSTFLAGS="${ORIGINAL_RUSTFLAGS} --cfg test_v128 --cfg test_v256" cargo_test ${1}
RUSTFLAGS="${ORIGINAL_RUSTFLAGS} --cfg test_v512" cargo_test ${1}

performs faster on travis.

---

The following things have already been tried:

• full split (one--cfg per command into 6 commands) and that performed worse (~55min for the testsuite)
• a split into two (16,32,64 + 128,256,512) but the memory consumption of that job was too large and the ios build triggered {i386,x86_64}-apple-ios targets pass too many thousands of arguments to the linker rust#52699.

So a split into three commands might be just what we need.

[gnzlbg]

The split has been working properly as is.