Correctly truncate error traces

[peterschrammel]

Fixes a regression introduced in #3795:
If a single counterexample violates several properties then the traces need to be correctly truncated for each property and printed in the same order as the properties are reported.

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[tautschnig]

Though weirdly not in all build configurations, this is failing the "several-threads" test:

File: /home/travis/build/diffblue/cbmc/src/goto-symex/build_goto_trace.cpp:288 function: build_goto_trace
Condition: last_step_to_keep == target.SSA_steps.end() || last_step_was_kept
Reason: last step in SSA trace to keep must not be filtered out as a sync instruction, not-taken branch, PHI node, or similar

[peterschrammel]

this is failing the "several-threads" test

A condition was missing in identifying the assertion step. Fixed now.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: cbd61b2).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/100599878