Bug 1678023 - [devtools] Force print.tab_modal.enabled to true in print.tab_modal.enabled r=ochameau

The preference print.tab_modal.enabled drives the usage of the print preview UI and is only set to true on EARLY_BETA_OR_EARLIER.
Force the pref in the test to avoid failures after late beta.

Differential Revision: https://phabricator.services.mozilla.com/D97448

devtools/client/framework/browser-toolbox/test/browser_browser_toolbox_print_preview.js

@@ -17,6 +17,10 @@ add_task(async function() {
   // Forces the Browser Toolbox to open on the inspector by default
   await pushPref("devtools.browsertoolbox.panel", "inspector");
 
+  // Force the modal print preview, otherwise the printPreview command will not
+  // open the expected UI.
+  await pushPref("print.tab_modal.enabled", true);
+
   // Open the tab *after* opening the Browser Toolbox in order to force creating the remote frames
   // late and exercise frame target watching code.
   await addTab(`data:text/html,<div id="test-div">PRINT PREVIEW TEST</div>`);