[lldb][test] Provide proper path to LLVM utils in API tests

In aea0668, API tests were supposed to be switched to LLVM tools
usage.
However, a path to an utility is made up incorrectly there: util name
should be prefixed with `llvm-`.

Hence it is fixed here.

Author: dzhidzhoev

lldb/packages/Python/lldbsuite/test/builders/builder.py

@@ -153,7 +153,9 @@ def getToolchainSpec(self, compiler):
         cc_dir = cc_path.parent
 
         def getToolchainUtil(util_name):
-            return os.path.join(configuration.llvm_tools_dir, util_name + exe_ext)
+            return os.path.join(
+                configuration.llvm_tools_dir, "llvm-" + util_name + exe_ext
+            )
 
         cxx = cc_dir / (cc_prefix + cxx_type + cc_ext)