CI: find and print backtraces

[EZoni]

I think this should work to find and print backtraces after all CI tests have run.

This is for Azure only. Local backtraces still need be inspected manually.

After trying many solutions, the current strategy is:

• avoid removing the backtrace files in the cleanup step of each test (we continue to remove all other files);
• have a separate workflow step to find and print the backtrace files in the Azure job (this is executed always).

The new Azure workflow step is labeled "Logs" and it comes right after the step labeled "Test".

[EZoni]

@WeiqunZhang @RemiLehe

Here you can find examples of how the backtrace files are displayed in the Azure log (by looking in the "Logs" step of a workflow, after "Test"): https://dev.azure.com/ECP-WarpX/WarpX/_build/results?buildId=19622&view=results.

These crashes were induced artificially by manual injection of NaN in the code.

It starts like this:

Starting: Logs
==============================================================================
Task         : Bash
Description  : Run a Bash script on macOS, Linux, or Windows
Version      : 3.246.1
Author       : Microsoft Corporation
Help         : https://docs.microsoft.com/azure/devops/pipelines/tasks/utility/bash
==============================================================================
Generating script.
========================== Starting Command Output ===========================
/usr/bin/bash /home/vsts/work/_temp/4ae82368-ed46-4960-8efa-c629bc4b2273.sh

Backtrace
---------
build/bin/test_2d_langmuir_multi_psatd_vay_deposition_particle_shape_4/Backtrace.0.0
---------
Host Name: fv-az23-266
=== If no file names and line numbers are shown below, one can run
            addr2line -Cpfie my_exefile my_line_address
    to convert `my_line_address` (e.g., 0x4a6b) into file name and line number.
    Or one can use amrex/Tools/Backtrace/parse_bt.py.

=== Please note that the line number reported by addr2line may not be accurate.
    One can use
            readelf -wl my_exefile | grep my_line_address'
    to find out the offset for that line.

 0: /home/vsts/work/1/s/build/lib/libamrex_2d.so(_ZN5amrex11BLBackTrace20print_backtrace_infoEP8_IO_FILE+0x4d) [0x7fbe6557805d]
    ?? ??:0

 1: /home/vsts/work/1/s/build/lib/libamrex_2d.so(_ZN5amrex11BLBackTrace7handlerEi+0x3e1) [0x7fbe6557a8a1]
    ?? ??:0

Let me know if you think we should format the header differently. The current header is

Backtrace
---------
build/bin/test_2d_langmuir_multi_psatd_vay_deposition_particle_shape_4/Backtrace.0.0
---------

followed by the backtrace.