tests: Add support for installed tests

They allow distrubtions to check whether libfprint continues working as
expected, in different contexts.

examples/meson.build

@@ -21,3 +21,8 @@ executable('cpp-test',
     'cpp-test.cpp',
     dependencies: libfprint_dep,
 )
+
+if installed_tests
+    install_subdir('prints',
+        install_dir: installed_tests_testdir)
+endif