Rename slowReporter to reflect that it is CommonJS (#4356)

Fix tests failing as a result of
https://github.com/matrix-org/matrix-js-sdk/pull/4355. This wasn't detected in
CI because the slowReporter is only enabled when building against develop.

jest.config.ts

@@ -39,7 +39,7 @@ if (env["GITHUB_ACTIONS"] !== undefined) {
 
     // if we're running against the develop branch, also enable the slow test reporter
     if (env["GITHUB_REF"] == "refs/heads/develop") {
-        reporters.push("<rootDir>/spec/slowReporter.js");
+        reporters.push("<rootDir>/spec/slowReporter.cjs");
     }
     config.reporters = reporters;
 }