test that -pp flags are correctly handled

(see ocaml/ocaml-lsp#1153)

Author: voodoos

tests/test-dirs/with-pp.t/run.t

@@ -0,0 +1,18 @@
+  $ cat >pp.sh <<EOF
+  > #!/bin/sh
+  > sed 's/world/universe/g' \$1
+  > EOF
+
+  $ chmod a+x pp.sh
+
+  $ cat >main.ml <<EOF
+  > type world!;;
+  > EOF
+
+  $ cat >.merlin <<EOF
+  > FLG -pp pp.sh
+  > EOF
+
+  $ $MERLIN single dump -what ppxed-source -filename main.ml<main.ml |
+  > tr -d '\n' | jq '.value' 
+  "type universe"