Make tests optional

--- a/meson_options.txt	2016-09-19 23:35:54.944511834 +0200
+++ b/meson_options.txt	2016-09-19 23:36:10.693485976 +0200
@@ -1,2 +1,3 @@
 option('enable-xunit', type : 'boolean', value : false)
 option('disable-help', type : 'boolean', value : false)
+option('enable-tests', type : 'boolean', value : false)
--- a/meson.build	2017-09-03 22:52:39.681974053 +0200
+++ b/meson.build	2017-09-03 22:53:29.934556188 +0200
@@ -71,8 +71,10 @@
 if not get_option('disable-help')
     subdir('help')
 endif
-subdir('tests')
-subdir('tests/validate-tests')
+if get_option('enable-tests')
+  subdir('tests')
+  subdir('tests/validate-tests')
+endif
 
 archiver = find_program('git-archive-all', required : false)
 if archiver.found()