test: isolate mempool event listeners in wallet-http-test

Also removed `assert.ok(false)` in these handlers since those
assertion errors would be swallowed by the mempool event
emitter and would not be caught by the test suite anyway.

These test modifications can be tested themselves by reviewers
by setting `broadcast: true`

test/wallet-http-test.js

@@ -252,13 +252,14 @@ describe('Wallet HTTP', function() {
   });
 
   it('should create an open and broadcast the tx', async () => {
+    let entered = false;
+    const handler = () => entered = true;
+    node.mempool.on('tx', handler);
+
     const json = await wallet.createOpen({
       name: name
     });
 
-    let entered = false;
-    node.mempool.on('tx', () => entered = true);
-
     // wait for tx event on mempool
     await common.event(node.mempool, 'tx');
 
@@ -269,20 +270,21 @@ describe('Wallet HTTP', function() {
 
     const opens = json.outputs.filter(output => output.covenant.type === types.OPEN);
     assert.equal(opens.length, 1);
+
+    // reset for next test
+    node.mempool.removeListener('tx', handler);
   });
 
   it('should create an open and not broadcast the transaction', async () => {
+    let entered = false;
+    const handler = () => entered = true;
+    node.mempool.on('tx', handler);
+
     const json = await wallet.createOpen({
       name: name,
       broadcast: false
     });
 
-    let entered = false;
-    node.mempool.on('tx', () => {
-      entered = true;
-      assert.ok(false);
-    });
-
     await sleep(500);
 
     // tx is not in the mempool
@@ -305,21 +307,22 @@ describe('Wallet HTTP', function() {
 
     const opens = mtx.outputs.filter(output => output.covenant.type === types.OPEN);
     assert.equal(opens.length, 1);
+
+    // reset for next test
+    node.mempool.removeListener('tx', handler);
   });
 
   it('should create an open and not sign the transaction', async () => {
+    let entered = false;
+    const handler = () => entered = true;
+    node.mempool.on('tx', handler);
+
     const json = await wallet.createOpen({
       name: name,
       broadcast: false,
       sign: false
     });
 
-    let entered = false;
-    node.mempool.on('tx', () => {
-      entered = true;
-      assert.ok(false);
-    });
-
     await sleep(500);
 
     // tx is not in the mempool
@@ -341,6 +344,9 @@ describe('Wallet HTTP', function() {
 
     // transaction not valid
     assert.equal(mtx.verify(), false);
+
+    // reset for next test
+    node.mempool.removeListener('tx', handler);
   });
 
   it('should throw error with incompatible broadcast and sign options', async () => {