tarfs_test: split large file test into independent cases

[siva]

Summary

This avoids globally set timeouts for the group of
tarfs_large tests on slower emulated architectures.

While here, lower each testcase's timeout to reflect
the reduction of work. On QEMU aarch64, the largest case
runs in ~500s on a modern desktop, so double that for a
conservative estimate.

[des]

Generating tests at run time (and especially generating them from atf_init_test_cases()) is problematic because the script gets run more than once per test case, so we keep generating code over and over even when we don't need it. I would prefer multiple thin hand-written wrappers around tarfs_large_test().