set TEST_CLEAN_DATABASE when running server tests (#806)

After adding a batch of new server tests, some interactions between
tests have shown up via a shared database. This sets an existing flag
for dealing with this problem, that is used during browser tests but
hadn't been needed before for server tests.

test/gen-server/migrations.ts

@@ -5,6 +5,7 @@ import {HomeDBManager} from 'app/gen-server/lib/HomeDBManager';
 import {Permissions} from 'app/gen-server/lib/Permissions';
 import {assert} from 'chai';
 import {addSeedData, createInitialDb, removeConnection, setUpDB} from 'test/gen-server/seed';
+import {EnvironmentSnapshot} from 'test/server/testUtils';
 
 import {Initial1536634251710 as Initial} from 'app/gen-server/migration/1536634251710-Initial';
 import {Login1539031763952 as Login} from 'app/gen-server/migration/1539031763952-Login';
@@ -63,11 +64,19 @@ function assertMembersGroup(org: Organization, exists: boolean) {
 }
 
 describe('migrations', function() {
+  let oldEnv: EnvironmentSnapshot;
 
   before(function() {
+    oldEnv = new EnvironmentSnapshot();
+    // This test is incompatible with TEST_CLEAN_DATABASE.
+    delete process.env.TEST_CLEAN_DATABASE;
     setUpDB(this);
   });
 
+  after(function() {
+    oldEnv.restore();
+  });
+
   beforeEach(async function() {
     await home.connect();
     await createInitialDb(home.connection, false);