Update image tests from placeholder.com to *.vercel.app (#38081)

Fixes flaky tests when placeholder.com is down

#38080 (comment)

Author: styfle

test/integration/image-component/unoptimized/pages/index.js

@@ -14,7 +14,7 @@ const Page = () => {
       <br />
       <Image
         id="external-image"
-        src="https://via.placeholder.com/800/000/FFF.png?text=test"
+        src="https://image-optimization-test.vercel.app/test.jpg"
         width={400}
         height={400}
       />

test/integration/image-component/unoptimized/test/index.test.js

@@ -61,23 +61,23 @@ function runTests() {
     await check(
       () =>
         browser.eval(`document.getElementById("external-image").currentSrc`),
-      /placeholder.com/
+      'https://image-optimization-test.vercel.app/test.jpg'
     )
     await check(
       () => browser.elementById('internal-image').getAttribute('src'),
-      /test\.png/
+      '/test.png'
     )
     await check(
       () => browser.elementById('static-image').getAttribute('src'),
       /test(.*)jpg/
     )
     await check(
       () => browser.elementById('external-image').getAttribute('src'),
-      /https:\/\/via\.placeholder\.com\/800\/000\/FFF\.png\?text=test/
+      'https://image-optimization-test.vercel.app/test.jpg'
     )
     await check(
       () => browser.elementById('eager-image').getAttribute('src'),
-      /test\.webp/
+      '/test.webp'
     )
 
     expect(

test/integration/image-future/unoptimized/pages/index.js

@@ -14,7 +14,7 @@ const Page = () => {
       <br />
       <Image
         id="external-image"
-        src="https://via.placeholder.com/800/000/FFF.png?text=test"
+        src="https://image-optimization-test.vercel.app/test.jpg"
         width={400}
         height={400}
       />

test/integration/image-future/unoptimized/test/index.test.js

@@ -27,7 +27,7 @@ function runTests() {
     ).toMatch(/test(.*)jpg/)
     expect(
       await browser.elementById('external-image').getAttribute('src')
-    ).toBe('https://via.placeholder.com/800/000/FFF.png?text=test')
+    ).toBe('https://image-optimization-test.vercel.app/test.jpg')
     expect(await browser.elementById('eager-image').getAttribute('src')).toBe(
       '/test.webp'
     )
@@ -61,7 +61,7 @@ function runTests() {
     await check(
       () =>
         browser.eval(`document.getElementById("external-image").currentSrc`),
-      /placeholder.com/
+      'https://image-optimization-test.vercel.app/test.jpg'
     )
 
     expect(
@@ -72,7 +72,7 @@ function runTests() {
     ).toMatch(/test(.*)jpg/)
     expect(
       await browser.elementById('external-image').getAttribute('src')
-    ).toBe('https://via.placeholder.com/800/000/FFF.png?text=test')
+    ).toBe('https://image-optimization-test.vercel.app/test.jpg')
     expect(await browser.elementById('eager-image').getAttribute('src')).toBe(
       '/test.webp'
     )