[test] Use full PATH lookup for tools

[mgorny]

Summary

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.

[labath]

lgtm, thanks.

lit/lit.cfg

 for pattern in [r"\bFileCheck\b",
                 r"\blldb-test\b",
                 r"\byaml2obj\b",
                 r"\| \bnot\b"]:
     tool_match = re.match(r"^(\\)?((\| )?)\W+b([0-9A-Za-z-_]+)\\b\W*$",
                           pattern)
     tool_pipe = tool_match.group(2)
     tool_name = tool_match.group(4)
-    tool_path = lit.util.which(tool_name, config.llvm_tools_dir)
+    tool_path = lit.util.which(tool_name, config.environment['PATH'])
     if not tool_path:
         # Warn, but still provide a substitution.
         lit_config.note(
-            'Did not find ' + tool_name + ' in ' + config.llvm_tools_dir)
+            'Did not find ' + tool_name + ' in ' + config.environment['PATH'])
     config.substitutions.append((pattern, tool_pipe + tool_path))
 
 # Shell execution
 if platform.system() not in ['Windows'] or lit_config.getBashPath() != '':
     config.available_features.add('shell')
 
 # Running on Darwin OS
 if platform.system() in ['Darwin']: