Testing some singleton implementations

[Stanislav Blinov]

You forgot to make the flag static for AtomicSingleton. I'd also 
move the timing into the threads themselves, for fairness :)

http://codepad.org/gvm3A88k

Timings on my machine:

ldc2 -unittest -release -O3:

Test time for LockSingleton: 537 msecs.
Test time for SyncSingleton: 2 msecs.
Test time for AtomicSingleton: 2.25 msecs.

dmd -unittest -release -O -inline:

Test time for LockSingleton: 451.5 msecs.
Test time for SyncSingleton: 7.75 msecs.
Test time for AtomicSingleton: 99.75 msecs.