Reduce thread count in ConcurrentHashMapTest to prevent system thread limit breach

[infomaniac777]

The earlier number of threads, 65536, is causing subprocess termination on Apple M3/16 GB/MacOS 15.1. This is understable since the OS thread limit per process is significantly lower(4096 as shown below).

sysctl kern.num_taskthreads
kern.num_taskthreads: 4096

Reducing the thread count to 1024 to add some some margin of error for other platforms as well. 1024 is practically already high enough for most real world use cases so this should be stress enough. But to somewhat compensate for the lower total thread count here, the number of iterations has been increased from 100 to 1000.

Its to be noted that other multi threaded tests in the same file are already using lower total thread counts(e.g. 32 here.

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