Check for non-empty stderr

Co-authored-by: Jon Ross-Perkins <[email protected]>

toolchain/testing/file_test.cpp

@@ -45,13 +45,12 @@ class ToolchainFileTest : public FileTestBase {
     }
 
     Driver driver(fs, &installation_, stdout, stderr);
-    auto loc_before = stderr.tell();
     auto driver_result = driver.RunCommand(test_args);
     // If any diagnostics have been produced, add a trailing newline to make the
     // last diagnostic match intermediate diagnostics (that have a newline
     // separator between them). This reduces churn when adding new diagnostics
     // to test cases.
-    if (loc_before != stderr.tell()) {
+    if (stderr.tell() > 0) {
       stderr << '\n';
     }