[9.20] chg: test: For TSAN builds, use libraries from /opt/tsan
The new TSAN images, the TSAN-enabled images install libraries to opt/tsan, synchronize the configure options and CFLAGS between gcc:tsan and clang:tsan images and set the PKG_CONFIG_PATH to /opt/tsan/lib.
Additionally, drop Debian bullseye that's EOL now.
Backport of MR !9324 (merged)