[lldb] [testsuite] Add split-file for check-lldb dependencies

D85968 started to use `split-file` and while buildbots run fine while
doing `make check-lldb` by hand I get:

.../llvm-monorepo-clangassert/tools/lldb/test/SymbolFile/DWARF/Output/DW_AT_declaration-with-children.s.script: line 2: split-file: command not found
failed:
  lldb-shell :: SymbolFile/DWARF/DW_AT_declaration-with-children.s

Differential Revision: https://reviews.llvm.org/D86144