[UBSAN] Fix UBSAN issue in test_bitcon_main.cpp

Summary:
This diff fixes an UBSAN issue introduced in D4841:

• It moves the master_test_suite_t reference to a variable;
• It adds an exception for the root cause of the UBSAN failure

Note that for some reason, this error only occurs with Clang < 9.

Test Plan: Run the build-ubsan configuration on CI.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4848