[CMake] Fix DynamicLibraryTests exports symbol when plugins are enabled (#102941)
Exporting symbols from tools directory never worked with
`LLVM_EXPORT_SYMBOLS_FOR_PLUGINS=On`. When that options is on, only
symbols from the static library is linked is exported due to the export
symbols computation script. DynamicLibraryTests needs to export a symbol
from the tools/executable directory, so update it to use export list
instead.