We now wait to split the input until each test actually runs. This means
some duplication of work between tests that previously used the same
input, but reduces startup overhead by not performing Vec copies. This
makes running a single test significantly faster without affecting the
total time to run all tests.

Before:

$ HTML5EVER_SRC_DIR=$(pwd) time -p ./build/html5ever-external-test --help > /dev/null
real         9.75
user         9.64
sys          0.10
$ HTML5EVER_SRC_DIR=$(pwd) time -p ./build/html5ever-external-test > /dev/null
real        17.72
user        60.12
sys          3.72

After:

$ HTML5EVER_SRC_DIR=$(pwd) time -p ./build/html5ever-external-test --help > /dev/null
real         0.51
user         0.49
sys          0.01
$ HTML5EVER_SRC_DIR=$(pwd) time -p ./build/html5ever-external-test > /dev/null
real        11.30
user        61.63
sys          4.00