Bug 1548118 Part 3 - Remove margin-bottom from column-balancing-nested-001.html and its reference. r=dbaron

Per bug 1487927, margin-bottom value is not always rendered as expected
with our column balancing algorithm. I'd like to remove it from
column-balancing-nested-001.html, and add <br> to separate each cases.

Differential Revision: https://phabricator.services.mozilla.com/D30287

--HG--
extra : moz-landing-system : lando

layout/reftests/columns/column-balancing-nested-001-ref.html

@@ -16,7 +16,6 @@
     -moz-column-count: 2;
     -moz-column-gap: 0px;
     -moz-column-rule: 1px solid black;
-    margin-bottom:1em;
     width:30ch;
   }
   .colset.lvl2 {
@@ -45,12 +44,14 @@
     <p>two two two two<br>two</p>
   </div>
 </div>
+<br>
 <div class="colset">
   <p>one one one one one<br>one</p>
   <div class="colset fixed-height lvl2">
     <p>two two two two two</p>
   </div>
 </div>
+<br>
 <div class="colset">
   <p>one<br>one</p>
   <div class="colset lvl2">

layout/reftests/columns/column-balancing-nested-001.html

@@ -16,7 +16,6 @@
     -moz-column-count: 2;
     -moz-column-gap: 0px;
     -moz-column-rule: 1px solid black;
-    margin-bottom:1em;
     width:30ch;
   }
   .colset.lvl2 {
@@ -41,12 +40,14 @@
     <p>two two two two two</p>
   </div>
 </div>
+<br>
 <div class="colset">
   <p>one one one one one<br>one</p>
   <div class="colset fixed-height lvl2">
     <p>two two two two two</p>
   </div>
 </div>
+<br>
 <div class="colset">
   <p>one<br>one</p>
   <div class="colset lvl2">