fix: fix linea ticker (#25436)

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## **Description**
Linea mainnet ticker should not be undefined.
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also include relevant motivation and context. Have in mind the following
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1. What is the reason for the change?
2. What is the improvement/solution?
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## **Related issues**

Fixes:

## **Manual testing steps**

1. Go to network selector and choose lieann
2. Go back to the wallet page
3. scam warning is displayed for Linea mainnet because ticker is
undefined

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**
![Screenshot 2024-06-20 at 13 14
37](https://github.com/MetaMask/metamask-extension/assets/26223211/a9a6dc6f-1dc3-4abe-a5df-f239e1a9420a)


<!-- [screenshots/recordings] -->

### **After**

![Screenshot 2024-06-20 at 13 13
22](https://github.com/MetaMask/metamask-extension/assets/26223211/42fcde95-3ccf-453c-b794-f89cda4d395c)

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- [x] I've completed the PR template to the best of my ability
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## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

ui/selectors/selectors.js

@@ -662,7 +662,7 @@ export const getNonTestNetworks = createDeepEqualSelector(
           imageUrl: LINEA_MAINNET_TOKEN_IMAGE_URL,
         },
         providerType: NETWORK_TYPES.LINEA_MAINNET,
-        ticker: TEST_NETWORK_TICKER_MAP[NETWORK_TYPES.LINEA_MAINNET],
+        ticker: CURRENCY_SYMBOLS.ETH,
         id: NETWORK_TYPES.LINEA_MAINNET,
         removable: false,
       },

ui/selectors/selectors.test.js

@@ -585,6 +585,38 @@ describe('Selectors', () => {
     });
   });
 
+  describe('#getNonTestNetworks', () => {
+    it('returns nonTestNetworks', () => {
+      const nonTestNetworks = selectors.getNonTestNetworks({
+        metamask: {
+          networkConfigurations: {},
+        },
+      });
+
+      // Assert that the `nonTestNetworks` array returned by the selector matches a specific structure.
+      expect(nonTestNetworks).toStrictEqual(
+        expect.arrayContaining([
+          expect.objectContaining({
+            chainId: expect.any(String),
+            nickname: expect.any(String),
+            rpcUrl: expect.any(String),
+            rpcPrefs: expect.objectContaining({
+              imageUrl: expect.any(String),
+            }),
+            providerType: expect.any(String),
+            ticker: expect.any(String),
+            id: expect.any(String),
+            removable: expect.any(Boolean),
+          }),
+        ]),
+      );
+
+      // Verify that each network object has a 'ticker' property that is not undefined
+      nonTestNetworks.forEach((network) => {
+        expect(network.ticker).toBeDefined();
+      });
+    });
+  });
   describe('#getAllNetworks', () => {
     it('sorts Localhost to the bottom of the test lists', () => {
       const networks = selectors.getAllNetworks({