[scalability testing] add json to track skipped journeys (#151629)

## Summary

Since developers are adding more api capacity tests, we need an easy way
to quickly skip the failing/unstable ones.

We run these tests on CI by passing path to the root test folder with
--journey-path flag:

0e18843e03/.buildkite/scripts/steps/scalability/benchmarking.sh (L82-L83)

The idea is that when there is a need to skip test, it should be added
to `x-pack/test/scalability/disabled_scalability_tests.json`:
```
[
   "x-pack/test/scalability/apis/api.telemetry.cluster_stats.json"
]
```

Using json array file seems like an easy option (similar to jest
config), but I'm open for suggestions.

src/dev/performance/run_scalability_cli.ts

@@ -40,13 +40,36 @@ run(
           })
       : [{ name: path.parse(journeyPath).name, path: journeyPath }];
 
+    const skippedFilePath = 'x-pack/test/scalability/disabled_scalability_tests.json';
+    const skipped: string[] = JSON.parse(
+      fs.readFileSync(path.resolve(REPO_ROOT, skippedFilePath), 'utf8')
+    ).map((relativePath: string) => path.resolve(REPO_ROOT, relativePath));
+    let filtered: Journey[] = [];
+
+    if (skipped.length === 0) {
+      filtered = journeys;
+    } else {
+      for (const journey of journeys) {
+        if (skipped.includes(journey.path)) {
+          log.warning(`Journey '${journey.name} is skipped'`);
+        } else {
+          filtered.push(journey);
+        }
+      }
+    }
+
+    if (filtered.length === 0) {
+      log.info(`No journeys found, check skipped list in '${skippedFilePath}'`);
+      return;
+    }
+
     log.info(
-      `Found ${journeys.length} journeys to run: ${JSON.stringify(journeys.map((j) => j.name))}`
+      `Found ${filtered.length} journeys to run: ${JSON.stringify(filtered.map((j) => j.name))}`
     );
 
     const failedJourneys = [];
 
-    for (const journey of journeys) {
+    for (const journey of filtered) {
       try {
         process.stdout.write(`--- Running scalability journey: ${journey.name}\n`);
         await runScalabilityJourney(journey.path, kibanaInstallDir);

x-pack/test/scalability/disabled_scalability_tests.json

@@ -0,0 +1,2 @@
+[
+]