perl: Allow running yath test in the build directory

For most purposes, the stock `ninja test` should be fine, but this
allows for doing other things with the `yath` during development.

perl/.yath.rc

@@ -1,2 +0,0 @@
-[test]
--I=rel(lib/Nix)

perl/.yath.rc.in

@@ -0,0 +1,2 @@
+[test]
+-I=rel(@lib_dir@)

perl/default.nix

@@ -26,7 +26,7 @@ perl.pkgs.toPerlModule (stdenv.mkDerivation (finalAttrs: {
       ./meson.build
       ./meson_options.txt
     ] ++ lib.optionals finalAttrs.doCheck [
-      ./.yath.rc
+      ./.yath.rc.in
       ./t
     ]);
   };

perl/meson.build

@@ -138,15 +138,23 @@ nix_perl_store_dep_list = [
 
 # # build
 # #-------------------------------------------------
-subdir('lib/Nix')
+lib_dir = join_paths('lib', 'Nix')
+subdir(lib_dir)
 
 if get_option('tests').enabled()
+  yath_rc_conf = configuration_data()
+  yath_rc_conf.set('lib_dir', lib_dir)
+  yath_rc = configure_file(
+    output : '.yath.rc',
+    input : '.yath.rc.in',
+    configuration : yath_rc_conf,
+  )
   subdir('t')
   test(
     'nix-perl-test',
     yath,
-    args : [
-      '-I=@0@'.format( join_paths( meson.current_build_dir(), 'lib', 'Nix') ),
-    ],
+    args : ['test'],
+    workdir : meson.current_build_dir(),
+    depends : [nix_perl_store_lib],
   )
 endif