PnetCDF mcoll_perf detects incorrect data

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The ``test/nonblocking/mcoll_perf.c`` test detects incorrect data when comparing two files that were written two different ways which should have identical content.
```
cd test/nonblocking
srun -n2 ./mcoll_perf /unifyfs/testfile.nc

P0: diff at line 282 variable[2] var1_2: NC_INT buf1 != buf2 at position 32762
```
After tracing ``pwrite`` and ``pread`` calls under a debugger, the problem is that both ranks write to the same byte offsets without any synchronization in between. In this case, rank 1 writes a fill value and rank 0 later writes actual data. It's a race as to which value actually ends up in the file.

The fill call is here:

https://github.com/Parallel-NetCDF/PnetCDF/blob/bb59553ca3542bc09ead12c6ce4e65b913ef51fa/test/nonblocking/mcoll_perf.c#L521

When filling the variable 2, rank 1 writes to (offset=648, length=8) and (offset=680, length=8).

And the write call is here:

https://github.com/Parallel-NetCDF/PnetCDF/blob/bb59553ca3542bc09ead12c6ce4e65b913ef51fa/test/nonblocking/mcoll_perf.c#L526

In that write, rank 0 writes to (offset=640, length=16) and (offset=672, length=16), which overlaps with the region that rank 1 wrote to during the fill operation.

The test case can be fixed by adding a call to ``ncmpi_sync(ncid);``:
```
for (i=2; i

Start with test/nonblocking/mcoll_perf.c at the fill loop around line 521 and the nonblocking collective writes around line 526. Run `cd test/nonblocking` and `srun -n2 ./mcoll_perf /unifyfs/testfile.nc` to reproduce the incorrect comparison, then verify that synchronization prevents overlapping fill and write operations from racing and the test completes without the reported data mismatch.

Written by the indexing model from the issue text.