Always Copy Backtraces (if any)

[ax3l]

For tests that crashed with backtraces but that were:

• not a restart test or were
• not doing a comparison on plotfiles

the backtraces were never copied. This should fix it.

[ax3l]

First seen in https://github.com/ECP-WarpX/WarpX/pull/2878

[zingale]

this probably fixes some of #111 , but I think somehow we are also missing copying the stdout for a test that crashes.