CRUD concurrency tests

See https://github.com/solid/solid-crud-tests/pull/47, check out that branch of the CRUD tests.
Run NSS in multi-user mode on localhost. Then:

export OIDC_ISSUER_ALICE=https://localhost:8443
export STORAGE_ROOT=https://alice.localhost:8443
export ALICE_WEBID=${STORAGE_ROOT}/profile/card#me

export USERNAME_ALICE=alice
export PASSWORD_ALICE=123
export CURL_RESULT_ALICE=`curl -ki $OIDC_ISSUER_ALICE/login/password -d"username=$USERNAME_ALICE&password=$PASSWORD_ALICE" | grep Set-Cookie`
export COOKIE_ALICE=`expr "$CURL_RESULT_ALICE" : '^Set-Cookie:\ \(.*\).'`

export NODE_TLS_REJECT_UNAUTHORIZED=0

./node_modules/.bin/jest test/surface/concurrency.test.ts

 FAIL  test/surface/concurrency.test.ts
  ● Concurrency › Try to create the same resource, using PUT 10 times › succeeds exactly once

    expect(received).toEqual(expected) // deep equality

    Expected: 1
    Received: 10

      88 |         results.filter((result) => responseCodeGroup(result.status) === "2xx")
      89 |           .length
    > 90 |       ).toEqual(1);
         |         ^
      91 |     });
      92 |     it("creates the resource", async () => {
      93 |       const result = await authFetcher.fetch(resourceUrl);

      at Object.<anonymous> (test/surface/concurrency.test.ts:90:9)

  ● Concurrency › Try to create the same resource, using PUT 10 times › emits websockets-pubsub on the container exactly once

    expect(received).toEqual(expected) // deep equality

    Expected: 1
    Received: 10

      115 |           (x) => x === `pub ${containerUrl}`
      116 |         ).length
    > 117 |       ).toEqual(1);
          |         ^
      118 |     });
      119 |     ifWps("emits websockets-pubsub on the resource exactly once", () => {
      120 |       expect(

      at Object.<anonymous> (test/surface/concurrency.test.ts:117:9)

  ● Concurrency › Try to create the same resource, using PUT 10 times › emits websockets-pubsub on the resource exactly once

    expect(received).toEqual(expected) // deep equality

    Expected: 1
    Received: 10

      122 |           (x) => x === `pub ${resourceUrl}`
      123 |         ).length
    > 124 |       ).toEqual(1);
          |         ^
      125 |     });
      126 |   });
      127 |   describe("Use PATCH 10 times to add triple to the same resource", () => {

      at Object.<anonymous> (test/surface/concurrency.test.ts:124:9)

Test Suites: 1 failed, 1 total
Tests:       3 failed, 6 passed, 9 total
Snapshots:   0 total
Time:        11.794 s
Ran all test suites matching /test\/surface\/concurrency.test.ts/i.

Check out the CRUD tests pull request branch, run NSS in multi-user mode on localhost with the listed environment variables, and execute test/surface/concurrency.test.ts. Start with the failing concurrency cases and compare the observed responses and websocket-pubsub events with the expectations. Done means the suite passes, including exactly one successful creation and one notification for each repeated PUT case.