tests: Search the appropriate directories for our GIR XML inputs

During "as-installed" testing, we should search the GIR_DIR for GIR XML,
instead of hard-coding that it is `${prefix}/share/gir-1.0`. This is
not the case on at least Debian, in order to make it possible to
install more than one architecture's flavour of `GLib-2.0.gir`,
which contains some architecture-specific `#define`s.

Also search GOBJECT_INTROSPECTION_DATADIR/GIR_SUFFIX (in practice
something like `/usr/share/gir-1.0` in all cases) to accommodate
distributions like Debian that move the architecture-independent
majority of GIR XML into /usr/share to avoid duplication, leaving
only the architecture-specific minority of files like `GLib-2.0.gir`
in the GIR_DIR.

Signed-off-by: Simon McVittie <smcv@collabora.com>

tests/lib/meson.build

@@ -0,0 +1,25 @@
+# Copyright 2024 Collabora Ltd.
+# Copyright 2025 Simon McVittie
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+install_data(
+  files(
+    'taptestrunner.py',
+    'testprogramrunner.py',
+  ),
+  install_dir: installed_tests_execdir,
+  install_tag: 'tests',
+)
+
+tests_conf = configuration_data()
+tests_conf.set('glib_datadir', glib_datadir)
+tests_conf.set('glib_girdir', glib_girdir)
+
+configure_file(
+  configuration: tests_conf,
+  input: 'glibconfig.py.in',
+  output: 'glibconfig.py',
+  install: installed_tests_enabled,
+  install_dir: installed_tests_execdir,
+  install_tag: 'tests',
+)