tests: fix use of deprecated meson methods

Fixes the following warnings

> tests/meson.build:123: WARNING: Project targets '>=0.56.0' but uses
  feature deprecated since '0.55.0': ExternalProgram.path. use
  ExternalProgram.full_path() instead
> tests/meson.build:124: WARNING: Project targets '>=0.56.0' but uses
  feature deprecated since '0.56.0': meson.source_root. use
  meson.project_source_root() or meson.global_source_root() instead.

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/meson.build

@@ -120,8 +120,8 @@ if not py.found()
   env += 'NO_PYTHON=1'
 else
   env += [
-    'PYTHON=' + py.path(),
+    'PYTHON=' + py.full_path(),
-    'PYTHONPATH=' + meson.source_root() / 'pylibfdt',
+    'PYTHONPATH=' + meson.project_source_root() / 'pylibfdt',
   ]
 endif
 if not yaml.found()