Fix missing lock in txvalidationcache_tests

Summary:
IsKeyInScriptCache() and AddKeyInScriptCache require holding
cs_main.

(bug introduced in D4834)

fixes T719 T720 T721

Test Plan:

cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug
ninja check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Maniphest Tasks: T721, T720, T719

Differential Revision: https://reviews.bitcoinabc.org/D5125