Don't panic when outputting TOML arrays in test failures.

TOML can't serialize arrays as the top level type.
Some of our test cases have TOML arrays as expected output.
Thus sometimes failed tests will panic when trying to report
expected and found output. Instead, we now manually "serialize"
TOML arrays.

tests/test-framework/src/error.rs

@@ -36,6 +36,23 @@ pub enum TestError {
     MissingTestConfig,
 }
 
+/// Make a user-readable string representing this Value.
+///
+/// TOML only supports tables as the top level type,
+/// so we handle arrays on our own.
+fn toml_to_string(x: &Value) -> String {
+    if let Some(array) = x.as_array() {
+        let mut s = String::new();
+        for value in array.iter() {
+            s.push_str(&toml::to_string(value).expect("serialization failed"));
+            s.push('\n');
+        }
+        s
+    } else {
+        toml::to_string(x).expect("serialization failed")
+    }
+}
+
 impl fmt::Display for TestError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let format_test =
@@ -50,8 +67,8 @@ impl fmt::Display for TestError {
                     "test #{}: {}expected\n{}\ngot\n{}",
                     index + 1,
                     format_test(test),
-                    toml::to_string(&expected).expect("serialization failed"),
-                    toml::to_string(&output).expect("serialization failed")
+                    toml_to_string(expected),
+                    toml_to_string(output),
                 )
             }
             TestError::PassedAndShouldntHave { test, index } => {