Cleanup use of ulimit in run tests.

Rather than setting a file size limit that is too low and overriding
the limit for all or many of the tests, set a single generous file
size limit. This should be much easier to maintain while still
providing some value.

Test: ./test/testrunner/testrunner.py --host --interpreter --jit --optimizing --relocate --no-relocate --prebuild --no-prebuild
Test: ./test/testrunner/testrunner.py --target
Change-Id: I9dc92c9db0819e8b65816b8be872dacc1871c30c