[8.6] [Cases] Fix flaky delete case test (#150476) (#150560)

# Backport

This will backport the following commits from `main` to `8.6`:
- [[Cases] Fix flaky delete case test
(#150476)](#150476)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Jonathan
Buttner","email":"56361221+jonathan-buttner@users.noreply.github.com"},"sourceCommit":{"committedDate":"2023-02-08T14:02:56Z","message":"[Cases]
Fix flaky delete case test (#150476)\n\nThis PR adds some `await` to
functions that might have been causing\r\nflakiness for some of our end
to end tests.\r\n\r\nFixes:
https://github.com/elastic/kibana/issues/145502\r\n\r\nFlaky test
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Fix flaky delete case test (#150476)\n\nThis PR adds some `await` to
functions that might have been causing\r\nflakiness for some of our end
to end tests.\r\n\r\nFixes:
https://github.com/elastic/kibana/issues/145502\r\n\r\nFlaky test
run:\r\nhttps://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/1878","sha":"9bf320f8f6a4552f96ea470679f3835d16f7e31b"}},"sourceBranch":"main","suggestedTargetBranches":["8.6"],"targetPullRequestStates":[{"branch":"main","label":"v8.7.0","labelRegex":"^v8.7.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/150476","number":150476,"mergeCommit":{"message":"[Cases]
Fix flaky delete case test (#150476)\n\nThis PR adds some `await` to
functions that might have been causing\r\nflakiness for some of our end
to end tests.\r\n\r\nFixes:
https://github.com/elastic/kibana/issues/145502\r\n\r\nFlaky test
run:\r\nhttps://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/1878","sha":"9bf320f8f6a4552f96ea470679f3835d16f7e31b"}},{"branch":"8.6","label":"v8.6.2","labelRegex":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"}]}]
BACKPORT-->

Co-authored-by: Jonathan Buttner <56361221+jonathan-buttner@users.noreply.github.com>

x-pack/test/functional/services/cases/api.ts

@@ -36,26 +36,23 @@ export function CasesAPIServiceProvider({ getService }: FtrProviderContext) {
         ...generateRandomCaseWithoutConnector(),
         ...overwrites,
       } as CasePostRequest;
-      const res = await createCaseAPI(kbnSupertest, caseData);
-      return res;
+
+      return createCaseAPI(kbnSupertest, caseData);
     },
 
     async createNthRandomCases(amount: number = 3) {
       const cases: CasePostRequest[] = Array.from(
         { length: amount },
         () => generateRandomCaseWithoutConnector() as CasePostRequest
       );
-      await pMap(
-        cases,
-        (caseData) => {
-          return createCaseAPI(kbnSupertest, caseData);
-        },
-        { concurrency: 4 }
-      );
+
+      await pMap(cases, async (caseData) => createCaseAPI(kbnSupertest, caseData), {
+        concurrency: 4,
+      });
     },
 
     async deleteAllCases() {
-      deleteAllCaseItems(es);
+      await deleteAllCaseItems(es);
     },
 
     async createAttachment({