Add set-pages-path tests

src/fixtures/gatsby/blank.expected.js

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+// Default Pages configuration for Gatsby
+module.exports = { pathPrefix: "/docs/" }

src/fixtures/gatsby/blank.js

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+// This file is not read by the test suite

src/fixtures/gatsby/default.expected.js

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+module.exports = {
+  pathPrefix: "/docs/",
+  siteMetadata: {
+    title: `My Gatsby Site`,
+    siteUrl: `https://www.yourdomain.tld`,
+  },
+  plugins: [],
+}

src/fixtures/gatsby/default.js

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+module.exports = {
+  siteMetadata: {
+    title: `My Gatsby Site`,
+    siteUrl: `https://www.yourdomain.tld`,
+  },
+  plugins: [],
+}