Fix OSX fail of tests due to too long path

This fixes the following error:

WARNING: IPC handle [...] is longer than 103 chars, try a shorter
--user-data-dir
Could not delete obsolete instance handle Error: ENOENT:
no such file or directory

This is a workaround for
microsoft/vscode#86382.
When using a long path for --user-data-dir, on OSX VS Code could
only be started once. Restarting VS Code would result in the
error given above.

test/runTests.ts

@@ -29,7 +29,7 @@ async function main(): Promise<void> {
     let tmpDirPath: string | undefined;
 
     try {
-      const tmpDirPrefix: string = Path.join(Path.dirname(vscodeExecutablePath), 'tmp-');
+      const tmpDirPrefix: string = Path.join(ltexDirPath, 'tmp-');
       console.log(`Creating temporary directory with prefix '${tmpDirPrefix}'...`);
       tmpDirPath = Fs.mkdtempSync(tmpDirPrefix);
       console.log(`Created temporary directory '${tmpDirPath}'.`);