meson: Exploit `tests` option to disable the test suite

Somehow we had the option, but no use of it to define the build.

Change-Id: I42431d962288bbc642413d77eafe754e48e599a6
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/meson.build b/meson.build
index 8ebf8af..c0a600d 100644
--- a/meson.build
+++ b/meson.build
@@ -89,4 +89,6 @@
            ],
            install: true)
 
-subdir('test')
+if get_option('tests')
+  subdir('test')
+endif