[test] Use full PATH lookup for tools

Use full PATH when looking up test tools rather than just llvm tools
directory. r320813 has added a lookup for 'lldb-test' which is part
of LLDB tools rather than LLVM, and therefore is not present
in llvm_tools_dir before LLDB is installed.

While technically we could introduce separate per-directory lookup
logic, there is no real reason not to use the PATH formed earlier here,
and this is what other tools are doing.

Differential Revision: https://reviews.llvm.org/D41726

llvm-svn: 321932