Fix --dump-cfg argument handling in run-test

This patch fixes a bug which was introduced in
9e73fcdbbdc302d9ed680662b0f86376c8baab10 when converting the run-test
script from bash to Python. The argument's value is now read as the
user-supplied string instead of a constant string literal "{arg}",
resulting in a correctly produced .cfg file.

Test: test-art-host, test-art-target.

Change-Id: I76e550edcfc66c9746e77d767e830412e0cf90cd
diff --git a/test/run-test b/test/run-test
index c59a4e3..4a6cd17 100755
--- a/test/run-test
+++ b/test/run-test
@@ -337,7 +337,7 @@
     elif arg == "--dump-cfg":
       shift()
       dump_cfg = "true"
-      dump_cfg_path = "{arg}"
+      dump_cfg_path = arg
       shift()
     elif arg == "--gdb":
       run_args += ["--gdb"]