Concurrent gtests take too long

On my machine, over more than half of the total runtime of gtests is currently used by just three tests:

- VirtualSpace.os_reserve_space_concurrent_vm
- VirtualSpace.os_virtual_space_concurrent_vm
- os_linux.reserve_memory_special_concurrent_vm

These tests do concurrent tests with 30 threads, with a timeout of 15 seconds each.

Since we run the gtests with several configurations (see hotspot/jtreg/gtests) these numbers multiply, which especially hurts when run as part of tier1. I think we can safely reduce the timeouts, still have good test coverage and recover some of the gtest runtime.