[Chronik] Take lock when accessing ActiveTip in chronikbridge_tests

Summary: Fixes a warning when compiling with clang

Test Plan:

cmake .. -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++  -DBUILD_BITCOIN_CHRONIK=1
ninja check

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D17201