start introducing data-testids for test usage

The Header rework exposed numerous test failures due to changes in structure.  
This could have been avoided if we had already been using data-testid attributes for the various things.

This starts small with the signin button.

Using "login" as the testid as this is what cloudbees had already been using in our custom header and the jelly is still loginLink

Author: jtnord

core/src/main/resources/hudson/security/SecurityRealm/loginLink.jelly

@@ -27,6 +27,7 @@ THE SOFTWARE.
 <j:jelly xmlns:j="jelly:core">
   <a class="jenkins-button"
      data-type="header-action"
+     data-testid="login"
      href="${rootURL}/${app.securityRealm.loginUrl}?from=${app.securityRealm.from}"
      style="aspect-ratio: unset; padding: 0.5rem 1rem"
      tooltip="${%signInTooltip}">