fix(tests): add more tests for '0 - something' case (postcss#88)

src/__tests__/index.js

@@ -118,6 +118,13 @@ test(
   'calc((100vw - 50em)/2)',
 );
 
+test(
+  'should reduce additions and subtractions (5)',
+  testValue,
+  'calc(10px - (100vw - 50em) / 2)',
+  'calc(10px - (100vw - 50em)/2)',
+);
+
 test(
   'should ignore value surrounding calc function (1)',
   testValue,
@@ -361,12 +368,33 @@ test(
 );
 
 test(
-  'should reduce substracted division expression from zero',
+  'should reduce substracted expression from zero (1)',
   testValue,
   'calc( 0 - (100vw - 10px) / 2 )',
   'calc((-100vw - -10px)/2)',
 );
 
+test(
+  'should reduce substracted expression from zero (2)',
+  testValue,
+  'calc( 0px - (100vw - 10px))',
+  'calc(-100vw - -10px)',
+);
+
+test(
+  'should reduce substracted expression from zero (3)',
+  testValue,
+  'calc( 0px - (100vw - 10px) * 2px )',
+  'calc((-100vw - -10px)*2px)',
+);
+
+test(
+  'should reduce substracted expression from zero (4)',
+  testValue,
+  'calc( 0px - (100vw + 10px))',
+  'calc(-100vw + -10px)',
+);
+
 test(
   'should reduce nested expression',
   testValue,