Add a test that ignores all hints then enables a lhs-rhs hint

Author: zliu41

README.md

@@ -324,7 +324,7 @@ Ignore directives can also be written in the hint files:
 
 These directives are applied in the order they are given, with later hints overriding earlier ones.
 
-You can choose to ignore all hints with `- ignore: {}` then selectively enable the ones you want (e.g. `- warn: {name: Use const}`), but it isn't a totally smooth experience (see [#747](https://github.com/ndmitchell/hlint/issues/747) and [#748](https://github.com/ndmitchell/hlint/issues/748)).
+You can choose to ignore all hints with `- ignore: {}` then selectively enable the ones you want (e.g. `- warn: {name: Use const}`), but it isn't a totally smooth experience (see [#748](https://github.com/ndmitchell/hlint/issues/748)).
 
 Finally, `hlint` defines the `__HLINT__` preprocessor definition (with value `1`), so problematic definitions (including those that don't parse) can be hidden with:
 

data/test-ignore-all-then-error.yaml

@@ -0,0 +1,5 @@
+- ignore: {}
+- error:
+    name: 'Only use one undefined'
+    lhs: 'undefined undefined'
+    rhs: 'undefined'

tests/ignore-all.test

@@ -0,0 +1,12 @@
+---------------------------------------------------------------------
+RUN tests/ignore-all-then-error.hs --hint=data/test-ignore-all-then-error.yaml
+FILE tests/ignore-all-then-error.hs
+main = undefined undefined
+OUTPUT
+tests/ignore-all-then-error.hs:1:8-26: Error: Only use one undefined
+Found:
+  undefined undefined
+Perhaps:
+  undefined
+
+1 error