[tests] Fix undefined fetch error for 'floating dependencies'

Pin whatwg-fetch dependence to '3.3.1'.

This fixes an error "fetch is not defined - maybe your browser targets are not covering everything you need?"
that shows up when running the tests for the floating dependencies stage.

That error comes from ember-fetch, see ember-cli/ember-fetch#547

package.json

@@ -100,6 +100,7 @@
   },
   "homepage": "https://Addepar.github.io/ember-table",
   "resolutions": {
+    "ember-cli-addon-docs/ember-fetch/whatwg-fetch": "3.3.1",
     "ember-cli-page-object": "1.15.1",
     "prettier": "1.18.2"
   }

yarn.lock

@@ -6004,7 +6004,7 @@ ember-fetch-adapter@^0.4.3:
   dependencies:
     ember-cli-babel "^6.12.0"
 
-ember-fetch@^6.2.0, ember-fetch@^6.4.0:
+ember-fetch@^6.2.0:
   version "6.7.1"
   resolved "https://registry.npmjs.org/ember-fetch/-/ember-fetch-6.7.1.tgz#778390bc9993d31cc32d429ae69a591e431e85ac"
   integrity sha512-B/s0HZWcIrDDz3wOxvAsWM2SyT4nND274aH3Othzxzax/lOJnGHKbNa+IGLrXKSja+ANeD5P8sVwDaAUw8pzpQ==
@@ -6022,6 +6022,24 @@ ember-fetch@^6.2.0, ember-fetch@^6.4.0:
     node-fetch "^2.6.0"
     whatwg-fetch "^3.0.0"
 
+ember-fetch@^6.4.0:
+  version "6.7.2"
+  resolved "https://registry.npmjs.org/ember-fetch/-/ember-fetch-6.7.2.tgz#82efce4a55a64863104347b71e598208b9acf518"
+  integrity sha512-+Dd++MJVkCXoqX2DPtFDjuoDMcLk+7fphLq7D8OoXwJq9KQMTff07sH18qhxWXV5Hqknvz3Uwy214g54vOboag==
+  dependencies:
+    abortcontroller-polyfill "^1.3.0"
+    broccoli-concat "^3.2.2"
+    broccoli-debug "^0.6.5"
+    broccoli-merge-trees "^3.0.0"
+    broccoli-rollup "^2.1.1"
+    broccoli-stew "^2.1.0"
+    broccoli-templater "^2.0.1"
+    calculate-cache-key-for-tree "^2.0.0"
+    caniuse-api "^3.0.0"
+    ember-cli-babel "^6.8.2"
+    node-fetch "^2.6.0"
+    whatwg-fetch "^3.0.0"
+
 ember-get-config@^0.2.4:
   version "0.2.4"
   resolved "https://registry.npmjs.org/ember-get-config/-/ember-get-config-0.2.4.tgz#118492a2a03d73e46004ed777928942021fe1ecd"
@@ -11834,7 +11852,12 @@ [email protected]:
   resolved "https://registry.npmjs.org/qs/-/qs-6.7.0.tgz#41dc1a015e3d581f1621776be31afb2876a9b1bc"
   integrity sha512-VCdBRNFTX1fyE7Nb6FYoURo/SPe62QCaAyzJvUjwRaIsc+NePBEniHlvxFmmX56+HZphIGtV0XeCirBtpDrTyQ==
 
-qs@^6.2.0, qs@^6.4.0:
+qs@^6.2.0:
+  version "6.9.4"
+  resolved "https://registry.npmjs.org/qs/-/qs-6.9.4.tgz#9090b290d1f91728d3c22e54843ca44aea5ab687"
+  integrity sha512-A1kFqHekCTM7cz0udomYUoYNWjBebHm/5wzU/XqrBRBNWectVH0QIiN+NEcZ0Dte5hvzHwbr8+XQmguPhJ6WdQ==
+
+qs@^6.4.0:
   version "6.8.0"
   resolved "https://registry.npmjs.org/qs/-/qs-6.8.0.tgz#87b763f0d37ca54200334cd57bb2ef8f68a1d081"
   integrity sha512-tPSkj8y92PfZVbinY1n84i1Qdx75lZjMQYx9WZhnkofyxzw2r7Ho39G3/aEvSUdebxpnnM4LZJCtvE/Aq3+s9w==
@@ -14485,10 +14508,10 @@ whatwg-encoding@^1.0.1, whatwg-encoding@^1.0.3, whatwg-encoding@^1.0.5:
   dependencies:
     iconv-lite "0.4.24"
 
-whatwg-fetch@^3.0.0:
-  version "3.0.0"
-  resolved "https://registry.npmjs.org/whatwg-fetch/-/whatwg-fetch-3.0.0.tgz#fc804e458cc460009b1a2b966bc8817d2578aefb"
-  integrity sha512-9GSJUgz1D4MfyKU7KRqwOjXCXTqWdFNvEr7eUBYchQiVc744mqK/MzXPNR2WsPkmkOa4ywfg8C2n8h+13Bey1Q==
+whatwg-fetch@3.3.1, whatwg-fetch@^3.0.0:
+  version "3.3.1"
+  resolved "https://registry.npmjs.org/whatwg-fetch/-/whatwg-fetch-3.3.1.tgz#6c1acf37dec176b0fd6bc9a74b616bec2f612935"
+  integrity sha512-faXTmGDcLuEPBpJwb5LQfyxvubKiE+RlbmmweFGKjvIPFj4uHTTfdtTIkdTRhC6OSH9S9eyYbx8kZ0UEaQqYTA==
 
 whatwg-mimetype@^2.1.0, whatwg-mimetype@^2.2.0:
   version "2.3.0"