Add UT cases for exprtk functions related to nan

Add unit test cases for below exprtk functions:
* maxIgnoreNaN
* sumIgnoreNaN
* ifNan

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I4a9ea4b1f88ad1fd76295f88778a5bfc6b4defd1
diff --git a/meson.build b/meson.build
index ca276f6..8265ab5 100644
--- a/meson.build
+++ b/meson.build
@@ -66,3 +66,8 @@
   configuration: conf_data,
   install: true,
   install_dir: systemd.get_variable('systemdsystemunitdir'))
+
+build_tests = get_option('tests')
+if not build_tests.disabled()
+  subdir('test')
+endif