Specify PPR test dimension via environment variable only

We can create separate Jest projects instead if there's
a desire to run a test with and without PPR.
But in this current state, the test was executed twice for each PPR state:
once in the PPR CI shard with PPR enabled and once in the
non-PPR CI shard with PPR enabled

test/e2e/app-dir/static-shell-debugging/static-shell-debugging.test.ts

@@ -1,59 +1,60 @@
 import { nextTestSetup } from 'e2e-utils'
 
 describe('static-shell-debugging', () => {
-  describe.each([
-    { ppr: true, debugging: true },
-    { ppr: false, debugging: false },
-  ])('ppr = $ppr, debugging = $debugging', (context) => {
-    const { next, skipped, isNextDev } = nextTestSetup({
-      files: __dirname,
-      // This test skips deployment because env vars that are doubled underscore prefixed
-      // are not supported. This is also intended to be used in development.
-      skipDeployment: true,
-      env: {
-        __NEXT_EXPERIMENTAL_STATIC_SHELL_DEBUGGING: context.debugging
-          ? '1'
-          : undefined,
-      },
-      nextConfig: {
-        experimental: { ppr: context.ppr },
-      },
-    })
+  const ppr = Boolean(process.env.__NEXT_EXPERIMENTAL_PPR)
+  const context = {
+    ppr,
+    debugging: ppr,
+  }
+
+  const { next, skipped, isNextDev } = nextTestSetup({
+    files: __dirname,
+    // This test skips deployment because env vars that are doubled underscore prefixed
+    // are not supported. This is also intended to be used in development.
+    skipDeployment: true,
+    env: {
+      __NEXT_EXPERIMENTAL_STATIC_SHELL_DEBUGGING: context.debugging
+        ? '1'
+        : undefined,
+    },
+    nextConfig: {
+      experimental: { ppr: context.ppr },
+    },
+  })
 
-    if (skipped) return
+  if (skipped) return
 
-    if (context.debugging && context.ppr) {
-      it('should only render the static shell', async () => {
-        const res = await next.fetch('/?__nextppronly=1')
-        expect(res.status).toBe(200)
+  if (context.debugging && context.ppr) {
+    it('should only render the static shell', async () => {
+      const res = await next.fetch('/?__nextppronly=1')
+      expect(res.status).toBe(200)
 
-        const html = await res.text()
-        expect(html).toContain('Fallback')
-        expect(html).not.toContain('Dynamic')
-      })
+      const html = await res.text()
+      expect(html).toContain('Fallback')
+      expect(html).not.toContain('Dynamic')
+    })
 
-      // The __nextppronly query param is currently only supported in dev mode.
-      if (isNextDev) {
-        it('should skip hydration to avoid blanking out the page', async () => {
-          const browser = await next.browser('/?__nextppronly=1', {
-            waitHydration: false,
-          })
+    // The __nextppronly query param is currently only supported in dev mode.
+    if (isNextDev) {
+      it('should skip hydration to avoid blanking out the page', async () => {
+        const browser = await next.browser('/?__nextppronly=1', {
+          waitHydration: false,
+        })
 
-          expect(await browser.elementByCss('div').text()).toBe('Fallback')
+        expect(await browser.elementByCss('div').text()).toBe('Fallback')
 
-          // Must not log the page error "Error: Connection closed."
-          expect(await browser.log()).toEqual([])
-        })
-      }
-    } else {
-      it('should render the full page', async () => {
-        const res = await next.fetch('/?__nextppronly=1')
-        expect(res.status).toBe(200)
-
-        const html = await res.text()
-        expect(html).toContain('Fallback')
-        expect(html).toContain('Dynamic')
+        // Must not log the page error "Error: Connection closed."
+        expect(await browser.log()).toEqual([])
       })
     }
-  })
+  } else {
+    it('should render the full page', async () => {
+      const res = await next.fetch('/?__nextppronly=1')
+      expect(res.status).toBe(200)
+
+      const html = await res.text()
+      expect(html).toContain('Fallback')
+      expect(html).toContain('Dynamic')
+    })
+  }
 })