[Security Solution] Fix flaky test for multiline diff algorithm (#201019

)

**Fixes: #201014
**Related to:** #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.

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({