vmTools/test: fix pcmanfm instead of pan

This goes much quicker. Also disable parallel build for machines with
many cores to avoid running out of memory.

pkgs/build-support/vm/test.nix

···
 
   buildHelloInVM = runInLinuxVM hello;
 
-  buildPanInVM = runInLinuxVM (pan // { memSize = 2048; });
-
+  buildPcmanrmInVM = runInLinuxVM (pcmanfm.overrideAttrs (old: {
+    # goes out-of-memory with many cores
+    enableParallelBuilding = false;
+  }));
 
   testRPMImage = makeImageTestScript diskImages.fedora27x86_64;