patch 8.2.0427: it is not possible to check for a typo in a feature name

Problem:    It is not possible to check for a typo in a feature name.
Solution:   Add an extra argument to has().
diff --git a/src/testdir/check.vim b/src/testdir/check.vim
index 6876101..0bbe7ca 100644
--- a/src/testdir/check.vim
+++ b/src/testdir/check.vim
@@ -6,6 +6,9 @@
 " Command to check for the presence of a feature.
 command -nargs=1 CheckFeature call CheckFeature(<f-args>)
 func CheckFeature(name)
+  if !has(a:name, 1)
+    throw 'Checking for non-existent feature ' .. a:name
+  endif
   if !has(a:name)
     MissingFeature a:name
   endif