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
diff --git a/meson.options b/meson.options
index 01f8577..5346c2b 100644
--- a/meson.options
+++ b/meson.options
@@ -5,3 +5,4 @@
     description: 'Enable modbus RTU.',
 )
 
+option('tests', type: 'feature', description: 'Build tests')
diff --git a/tests/meson.build b/tests/meson.build
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/tests/meson.build
@@ -0,0 +1 @@
+