Mark tests that timeout on ubuntu 18.04 job as KNOWNBUG

[NlightNFotis]

Recently a number of PRs have been failing the Ubuntu 18.04 package build PR job, with a mysterious 137 exit code that seems to be indicating a out-of-memory SIGKILL received.

This PR is deactivating the failing tests so that the latest release can go forward while we take some more time to investigate this.

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[codecov]

Codecov Report

Merging #7133 (bfdd5ac) into develop (13e2eca) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

❗ Current head bfdd5ac differs from pull request most recent head 63033ec. Consider uploading reports for the commit 63033ec to get more accurate results

@@             Coverage Diff             @@
##           develop    #7133      +/-   ##
===========================================
- Coverage    77.87%   77.86%   -0.01%     
===========================================
  Files         1576     1576              
  Lines       181801   181808       +7     
===========================================
- Hits        141573   141568       -5     
- Misses       40228    40240      +12     

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_expr.cpp	74.17% <0.00%> (ø)
src/util/pointer_expr.h	80.83% <0.00%> (-1.74%)	⬇️
jbmc/src/java_bytecode/java_bytecode_language.cpp	92.24% <0.00%> (-0.58%)	⬇️
jbmc/src/jbmc/jbmc_parse_options.cpp	75.65% <0.00%> (-0.24%)	⬇️

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[NlightNFotis]

Given that #7134 has the same aims and has been merged already, this is now redundant.