Don't shutdown gl in headless so quickly.

Oops.
Just disable PPGe during tests, should be okay.
Try to force test output to stdout.
Don't zero initialize opcode.

This slows down JitBlockCache::Init() a lot, which impacts test speed more
than you'd expect.
Optionally read test filenames from stdin.

This way we don't hit command line length limits.
Merge pull request #3823 from unknownbrackets/headless

Fix headless on buildbot (command line length), speed up further