[Security Solution] Fix flaky test for multiline diff algorithm (elas…

…tic#201019)

**Fixes: elastic#201014
**Related to:** elastic#199388

## Summary

This PR increases the threshold (time limit) value for the test by 2x
from 500 ms to 1000 ms. Hope it should be enough to eliminate flakiness
on CI.

(cherry picked from commit c9e7820)

x-pack/plugins/security_solution/server/lib/detection_engine/prebuilt_rules/logic/diff/calculation/algorithms/multi_line_string_diff_algorithm.test.ts

@@ -174,9 +174,11 @@ describe('multiLineStringDiffAlgorithm', () => {
       const result = multiLineStringDiffAlgorithm(mockVersions);
       const endTime = performance.now();
 
-      // If the regex merge in this function takes over 500ms, this test fails
+      // If the regex merge in this function takes over 1 sec, this test fails
       // Performance measurements: https://github.com/elastic/kibana/pull/199388
-      expect(endTime - startTime).toBeLessThan(500);
+      // NOTE: despite the fact that this test runs in ~50ms locally, on CI it
+      // runs slower and can be flaky even with a 500ms threshold.
+      expect(endTime - startTime).toBeLessThan(1000);
 
       expect(result).toEqual(
         expect.objectContaining({