[kbn-test] improve check_ftr_configs script (#189357)

## Summary

Follow-up to #188854

On CI script is taking >1 min, while before it was taking seconds. 
Probably because 80+ files were loaded as potential FTR configs. I
adjusted regular expressions to minimize the amount of files that we
need to load to validate if it is FTR config or not.

In this CI run script took 20s (part of `Quick Checks` group)

packages/kbn-test/src/functional_test_runner/lib/config/run_check_ftr_configs_cli.ts

@@ -41,6 +41,8 @@ export async function runCheckFtrConfigsCli() {
         .split('\n')
         .map((file) => Path.resolve(REPO_ROOT, file));
 
+      const loadingConfigs = [];
+
       const possibleConfigs = files.filter((file) => {
         if (IGNORED_PATHS.includes(file)) {
           return false;
@@ -66,24 +68,45 @@ export async function runCheckFtrConfigsCli() {
           return false;
         }
 
-        if (file.match(/mocks.ts$/)) {
+        // No FTR configs in /packages/
+        if (file.match(/\/packages\//)) {
+          return false;
+        }
+
+        if (file.match(/(mock|mocks).ts$/)) {
           return false;
         }
 
         const fileContent = readFileSync(file).toString();
 
-        if (fileContent.match(/(testRunner)|(testFiles)/)) {
+        if (
+          // explicitly define 'testRunner' or 'testFiles'
+          fileContent.match(/(testRunner)|(testFiles)/) ||
+          // export default createTestConfig
+          fileContent.match(/export\s+default\s+createTestConfig/) ||
+          // export default async function ({ readConfigFile }: FtrConfigProviderContext)
+          // async function config({ readConfigFile }: FtrConfigProviderContext)
+          // export default async function (ftrConfigProviderContext: FtrConfigProviderContext)
+          fileContent.match(
+            /(?:export\s+default\s+)?async\s+function(?:\s+\w+)?\s*\(\s*(?:\{\s*readConfigFile\s*\}|\w+)\s*(?::\s*FtrConfigProviderContext\s*)?\)/
+          )
+        ) {
           // test config
           return true;
         }
 
-        if (fileContent.match(/(describe)|(defineCypressConfig)/)) {
+        if (file.match(/config.ts$/) && fileContent.match(/export\s+default\s+configs\./)) {
+          return true;
+        }
+
+        if (fileContent.match(/(describe)|(defineCypressConfig)|(cy\.)/)) {
           // test file or Cypress config
           return false;
         }
 
         // FTR config file should have default export
         try {
+          loadingConfigs.push(file);
           // eslint-disable-next-line @typescript-eslint/no-var-requires
           const exports = require(file);
           const defaultExport = exports.__esModule ? exports.default : exports;
@@ -94,6 +117,10 @@ export async function runCheckFtrConfigsCli() {
         }
       });
 
+      if (loadingConfigs.length) {
+        log.info(`${loadingConfigs.length} files were loaded as FTR configs for validation`);
+      }
+
       const { allFtrConfigs, manifestPaths } = getAllFtrConfigsAndManifests();
 
       const invalid = possibleConfigs.filter((path) => !allFtrConfigs.includes(path));