tests: Remove no longer needed UBSan suppressions (issues fixed). Add documentation.

Summary: This is a backport of Core PR17209

Test Plan:

cmake -GNinja ..   -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DENABLE_SANITIZERS="undefined"
UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=0 ninja check check-functional
grep -i "runtime error" test/log/*.log

Check that there are no errors for wallet.cpp or fs.cpp

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8188