[fusl] Add llvm's lit tool to third_party

third_party/lit/tests/test-output.py

+# RUN: %{lit} -j 1 -v %{inputs}/test-data --output %t.results.out > %t.out
+# RUN: FileCheck < %t.results.out %s
+
+# CHECK: {
+# CHECK: "__version__"
+# CHECK: "elapsed"
+# CHECK-NEXT: "tests": [
+# CHECK-NEXT:   {
+# CHECK-NEXT:     "code": "PASS",
+# CHECK-NEXT:     "elapsed": {{[0-9.]+}},
+# CHECK-NEXT:     "metrics": {
+# CHECK-NEXT:       "value0": 1,
+# CHECK-NEXT:       "value1": 2.3456
+# CHECK-NEXT:     }
+# CHECK-NEXT:     "name": "test-data :: metrics.ini",
+# CHECK-NEXT:     "output": "Test passed."
+# CHECK-NEXT:   }
+# CHECK-NEXT: ]
+# CHECK-NEXT: }