Test cases for bugs spotted in MPR7873

This ensures that Topdirs.opt_directory works both in a compiled
toplevel both when used interactively (GPR#1041 regression) and when
called with a script argument (broken since the dawn of time)

testsuite/tests/tool-toplevel/ocamltests

@@ -2,6 +2,7 @@ exotic_lists.ml
 pr6468.ml
 pr7060.ml
 pr7751.ml
+pr7873.ml
 strings.ml
 tracing.ml
 error_highlighting.ml

testsuite/tests/tool-toplevel/pr7873.ml

@@ -0,0 +1,26 @@
+(* TEST
+   files = "pr7873.sh"
+  * setup-ocamlc.byte-build-env
+  ** script
+   script = "mkdir hello"
+  *** script
+   script = "cp ${test_source_directory}/pr7873_hello.ml hello/hello.ml"
+  **** ocamlc.byte
+   module = "hello/hello.ml"
+  ***** ocamlc.byte
+   module = ""
+   program = "${test_build_directory}/mytop.exe"
+   include ocamlcommon
+   libraries += "ocamlbytecomp ocamltoplevel"
+   flags = "-I hello"
+   all_modules = "pr7873.ml hello.cmo topstart.cmo"
+  ****** script
+   script = "sh ./pr7873.sh"
+  ******* check-program-output
+  ****** script
+   script = "sh ./pr7873.sh direct"
+  ******* check-program-output
+   reference = "${test_source_directory}/pr7873a.reference"
+*)
+
+let () = Topdirs.dir_directory "hello"

testsuite/tests/tool-toplevel/pr7873.reference

@@ -0,0 +1,2 @@
+42- : unit = ()
+

testsuite/tests/tool-toplevel/pr7873.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+if [ $# -gt 0 ] ; then
+  echo 'print_int Hello.hello' > test.ml
+  ${ocamlrun} ./mytop.exe -noinit -nostdlib -I ${ocamlsrcdir}/stdlib test.ml
+else
+  echo 'print_int Hello.hello;;' | ${ocamlrun} ./mytop.exe -noinit -nostdlib -I ${ocamlsrcdir}/stdlib -no-version -noprompt
+fi
+exit 0

testsuite/tests/tool-toplevel/pr7873_hello.ml

@@ -0,0 +1 @@
+let hello = 42

testsuite/tests/tool-toplevel/pr7873a.reference

@@ -0,0 +1 @@
+42