[EuiTableRowCell] Fix where className is applied to ⚠️

- I have no idea why it would apply to the content div instead of to the actual cell element :|||

- !! this is a breaking DOM change and will need careful Kibana migration

src/components/table/__snapshots__/table_row_cell.test.tsx.snap

@@ -86,11 +86,11 @@ exports[`renders EuiTableRowCell 1`] = `
     <tr>
       <td
         aria-label="aria-label"
-        class="euiTableRowCell euiTableRowCell--middle emotion-euiTableRowCell-desktop"
+        class="euiTableRowCell testClass1 testClass2 euiTableRowCell--middle emotion-euiTableRowCell-desktop-euiTestCss"
         data-test-subj="test subject string"
       >
         <div
-          class="euiTableCellContent testClass1 testClass2 emotion-euiTestCss"
+          class="euiTableCellContent"
         >
           <span
             class="euiTableCellContent__text"

src/components/table/table_row_cell.tsx

@@ -146,15 +146,15 @@ export const EuiTableRowCell: FunctionComponent<Props> = ({
       : [styles.desktop]),
   ];
 
-  const cellClasses = classNames('euiTableRowCell', {
+  const cellClasses = classNames('euiTableRowCell', className, {
     'euiTableRowCell--hasActions': hasActions,
     'euiTableRowCell--isExpander': isExpander,
     'euiTableRowCell--hideForDesktop': mobileOptions.only,
     'euiTableRowCell--enlargeForMobile': mobileOptions.enlarge,
     [`euiTableRowCell--${valign}`]: valign,
   });
 
-  const contentClasses = classNames('euiTableCellContent', className, {
+  const contentClasses = classNames('euiTableCellContent', {
     'euiTableCellContent--alignRight': align === RIGHT_ALIGNMENT,
     'euiTableCellContent--alignCenter': align === CENTER_ALIGNMENT,
     'euiTableCellContent--showOnHover': showOnHover,
@@ -164,7 +164,7 @@ export const EuiTableRowCell: FunctionComponent<Props> = ({
     'euiTableCellContent--overflowingContent': textOnly !== true,
   });
 
-  const mobileContentClasses = classNames('euiTableCellContent', className, {
+  const mobileContentClasses = classNames('euiTableCellContent', {
     'euiTableCellContent--alignRight':
       mobileOptions.align === RIGHT_ALIGNMENT || align === RIGHT_ALIGNMENT,
     'euiTableCellContent--alignCenter':