tests: add dir and meson option

Add tests dir and meson option to enable tests.

Tested: meson build passed.

Change-Id: I8fce7227664e3ae9d41d25a8d22196baad59ed20
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/meson.build b/meson.build
index 9bd3146..cbc46df 100644
--- a/meson.build
+++ b/meson.build
@@ -22,3 +22,7 @@
 if get_option('modbus-rtu').allowed()
     subdir('rtu')
 endif
+
+if get_option('tests').allowed()
+    subdir('tests')
+endif