test: Add valgrind suppression for libuv memory leak

A memory leak happens in uv_spawn when stdio is set to UV_IGNORE and the child
process fails to start. This only happens when libuv is compiled with gcc and
optimizations enabled(the default).

Compiling with '-O0' removes the leak, but all that can be done right now is
ignore the leak by adding a valgrind suppression.

.valgrind.supp

@@ -30,3 +30,10 @@
    fun:vim_strsave
    fun:ex_function
 }
+{
+  uv_spawn_with_optimizations
+  Memcheck:Leak
+  fun:malloc
+  fun:uv_spawn
+  fun:job_start
+}