Use an absolute path in test_process_output_fail_to_start

This test is designed to ensure that running a non-existent executable
results in a correct error message (FileNotFound in this case of this
test).  However, if you try to run an executable that doesn't exist, and
that requires searching through the $PATH, and one of the $PATH components
is not readable, then a PermissionDenied error will be returned, instead
of FileNotFound.

src/libstd/run.rs

@@ -360,7 +360,7 @@ mod tests {
             trapped_io_error = true;
             assert_eq!(e.kind, FileNotFound);
         }).inside(|| -> Option<run::ProcessOutput> {
-            run::process_output("no-binary-by-this-name-should-exist", [])
+            run::process_output("/no-binary-by-this-name-should-exist", [])
         });
         assert!(trapped_io_error);
         assert!(opt_outp.is_none());