gcc.dg/asan/asan.exp checks if asan_init succeeds before running anything, and likewise for gcc.dg/ubsan/ubsan.exp.  Success is determined in asan_init as meaning that a nonempty set of link flags was produced.  But for installed toolchain testing, the logic for finding like flags won't find any, and this doesn't matter - the library will already be installed where the installed compiler can find it, and LD_LIBRARY_PATH is the board file's responsibility.  Thus, the right way to test for success is whether a test file can be successfully linked with the determined options, not whether the determined options are nonempty.