fix!: don't panic, instead provide an error when fixture script fails.

This makes introspection easier, even though we still have to print to
script output to stderr in order to make it legible.

Author: Byron

tests/tools/src/lib.rs

@@ -583,13 +583,10 @@ fn scripted_fixture_read_only_with_args_inner(
                 };
                 if !output.status.success() {
                     write_failure_marker(&failure_marker);
+                    eprintln!("stdout: {}", output.stdout.as_bstr());
+                    eprintln!("stderr: {}", output.stderr.as_bstr());
+                    return Err(format!("fixture script of {cmd:?} failed").into());
                 }
-                assert!(
-                    output.status.success(),
-                    "fixture script of {cmd:?} failed: stdout: {}\nstderr: {}",
-                    output.stdout.as_bstr(),
-                    output.stderr.as_bstr()
-                );
                 create_archive_if_we_should(
                     &script_result_directory,
                     &archive_file_path,