fix(Masthead): have nav item render with selected styles in mobile nav (

carbon-design-system#4259)

### Related Ticket(s)

menu item doesn't show active state in hamburg menu in masthead carbon-design-system#4076

### Description

The `LeftNav` menu item wasn't checking with the proper prop, so it wasn't detecting whether to render the nav item as selected in mobile.

FIXED:
<img width="652" alt="Screen Shot 2020-10-19 at 4 26 13 PM" src="https://user-images.githubusercontent.com/54281166/96508017-fcd87200-1227-11eb-98d1-3b56c314b5a1.png">

### Changelog

**New**

- add `titleEnglish` field to the `Nulla quis sem at nibh elementum imperdiet` custom nav item to allow for additional testing

**Changed**

- check for `rest.selectedMenuItem` in the `LeftNav` and render `#{$prefix}--masthead__l0-nav--selected-item` classname - this is specific for L0 nav items that don't have a menu


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<!-- *** "package: services": Services -->
<!-- *** "package: utilities": Utilities -->
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packages/react/src/__tests__/__snapshots__/storyshots.test.js.snap

@@ -93081,6 +93081,7 @@ exports[`Storyshots Components|Masthead With L 1 1`] = `
                     "hasMenu": false,
                     "menuSections": Array [],
                     "title": "Nulla quis sem at nibh elementum imperdiet",
+                    "titleEnglish": "Nulla quis sem at nibh elementum imperdiet",
                     "url": "https://www.ibm.com/industries?lnk=min",
                   },
                   Object {
@@ -93406,6 +93407,7 @@ exports[`Storyshots Components|Masthead With L 1 1`] = `
                 "hasMenu": false,
                 "menuSections": Array [],
                 "title": "Nulla quis sem at nibh elementum imperdiet",
+                "titleEnglish": "Nulla quis sem at nibh elementum imperdiet",
                 "url": "https://www.ibm.com/industries?lnk=min",
               },
               Object {
@@ -94066,6 +94068,7 @@ exports[`Storyshots Components|Masthead With L 1 1`] = `
                         "hasMenu": false,
                         "menuSections": Array [],
                         "title": "Nulla quis sem at nibh elementum imperdiet",
+                        "titleEnglish": "Nulla quis sem at nibh elementum imperdiet",
                         "url": "https://www.ibm.com/industries?lnk=min",
                       },
                       Object {

packages/react/src/components/Masthead/MastheadLeftNav.js

@@ -127,7 +127,8 @@ const MastheadLeftNav = ({
         <SideNavLink
           href={link.url}
           className={
-            rest.selected && `${prefix}--masthead__side-nav--submemu--selected`
+            rest.selectedMenuItem === link.titleEnglish &&
+            `${prefix}--masthead__side-nav--submemu--selected`
           }
           data-autoid={`${stablePrefix}--masthead-${rest.navType}-sidenav__l0-nav${i}`}
           key={i}>

packages/react/src/components/Masthead/MastheadTopNav.js

@@ -61,7 +61,7 @@ const MastheadTopNav = ({ navigation, ...topNavProps }) => {
     } else {
       return (
         <HeaderMenuItem
-          className={selected && `${prefix}--masthead__l0-nav--selected`}
+          className={selected && `${prefix}--masthead__l0-nav--selected-item`}
           href={link.url}
           data-autoid={autoid}
           key={i}>

packages/react/src/components/Masthead/__stories__/data/MastheadLinks.js

@@ -159,6 +159,7 @@ const mastheadLinks = [
   },
   {
     title: 'Nulla quis sem at nibh elementum imperdiet',
+    titleEnglish: 'Nulla quis sem at nibh elementum imperdiet',
     url: 'https://www.ibm.com/industries?lnk=min',
     hasMenu: false,
     hasMegapanel: false,

packages/styles/scss/components/masthead/_masthead.scss

@@ -174,7 +174,8 @@ $search-transition-timing: 95ms;
     }
   }
 
-  .#{$prefix}--masthead__l0-nav--selected a[aria-expanded='false'] {
+  .#{$prefix}--masthead__l0-nav--selected a[aria-expanded='false'],
+  .#{$prefix}--masthead__l0-nav--selected-item a {
     border-bottom: solid 3px $interactive-01;
   }