writing plotfiles can fail when running test suite in parallel

[BenWibking]

Describe the bug
Outputs from plotfiles can collide when running them simultaneously with ctest -jN with N > 1.

AMReX is not designed to write outputs from multiple simulations in the same directory. In practice, however, this only seems to be an issue on parallel filesystems where metadata operations are slow.

The easiest fix is to set a unique prefix for each test problem in its input file, e.g.:

# default is "plt"
plotfile_prefix = mytest_plt

# default is "chk"
checkpoint_prefix = mytest_chk

To Reproduce
Steps to reproduce the behavior:

1. Run the test suite on a Lustre filesystem
2. See test failures due to std::rename failing.

Additional context
This seems to happen on Gadi.

[chongchonghe]

I'm happy to take on this.