runInMachine: Support passAsFile

We need to rewrite attributes passed via files to their location in
/tmp/xchg in the VM. Otherwise functions like runCommand don't work.

nixos/lib/testing.nix

@@ -157,9 +157,7 @@ rec {
         ${coreutils}/bin/mkdir -p $TMPDIR
         cd $TMPDIR
 
-        $origBuilder $origArgs
+        exec $origBuilder $origArgs
-
-        exit $?
       '';
 
       testScript = ''
@@ -172,9 +170,20 @@ rec {
       '';
 
       vmRunCommand = writeText "vm-run" ''
+        xchg=vm-state-client/xchg
         ${coreutils}/bin/mkdir $out
-        ${coreutils}/bin/mkdir -p vm-state-client/xchg
+        ${coreutils}/bin/mkdir -p $xchg
-        export > vm-state-client/xchg/saved-env
+
+        for i in $passAsFile; do
+          i2=''${i}Path
+          _basename=$(${coreutils}/bin/basename ''${!i2})
+          ${coreutils}/bin/cp ''${!i2} $xchg/$_basename
+          eval $i2=/tmp/xchg/$_basename
+          ${coreutils}/bin/ls -la $xchg
+        done
+        unset i i2 _basename
+
+        export > $xchg/saved-env
         export tests='${testScript}'
         ${testDriver}/bin/nixos-test-driver ${vm.config.system.build.vm}/bin/run-*-vm
       ''; # */