mdtest: print out informational output only once

[bertschinger]

When informational output is printed about the arguments passed to mdtest, previously every rank printed out the same messages. Duplicating the same informational messages N times for N ranks isn't helpful for the user.

This change makes it so that only rank 0 prints out informational messages about the parameters.

Before:

$ mpirun -n 4 ./src/mdtest -d test_dir
WARNING: Read bytes is 0, thus, a read test will actually just open/close
WARNING: Read bytes is 0, thus, a read test will actually just open/close
WARNING: Read bytes is 0, thus, a read test will actually just open/close
-- started at 12/10/2025 10:15:46 --

mdtest-4.1.0+dev was launched with 4 total task(s) on 1 node(s)
Command line used: ./src/mdtest '-d' 'test_dir'
WARNING: Read bytes is 0, thus, a read test will actually just open/close
...

After:

$ mpirun -n 4 ./src/mdtest -d test_dir
-- started at 12/10/2025 10:13:29 --

mdtest-4.1.0+dev was launched with 4 total task(s) on 1 node(s)
Command line used: ./src/mdtest '-d' 'test_dir'
WARNING: Read bytes is 0, thus, a read test will actually just open/close
...

[JulianKunkel]

Cleaner, good job!