(core) repair a migration test when running against postgres

Summary:
This is fixing some fall-out from some environmental changes after
another batch of tests were moved to grist-core.

Test Plan: this is fixing a test, no app changes

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D4156

test/gen-server/migrations.ts

@@ -79,6 +79,13 @@ describe('migrations', function() {
 
   beforeEach(async function() {
     await home.connect();
+    // If testing against postgres, remove all tables.
+    // If SQLite, we're using a fresh in-memory db each time.
+    const sqlite = home.connection.driver.options.type === 'sqlite';
+    if (!sqlite) {
+      await home.connection.query('DROP SCHEMA public CASCADE');
+      await home.connection.query('CREATE SCHEMA public');
+    }
     await createInitialDb(home.connection, false);
   });