Auto merge of #8692 - yaymukund:improve-tests-json-diffing, r=alexcrichton

Display formatted output for JSON diffing in tests.

This affects all tests that use `validate_upload`.

Before
<img src="https://user-images.githubusercontent.com/590450/92583169-da972000-f289-11ea-9f27-f09071a023b1.png" width="400">

After
<img src="https://user-images.githubusercontent.com/590450/92583195-e2ef5b00-f289-11ea-9ee5-a32f630a6472.png" width="400">

Author: bors

crates/cargo-test-support/src/publish.rs

@@ -75,8 +75,10 @@ fn _validate_upload(
     f.read_exact(&mut json_bytes).expect("read JSON data");
     let actual_json = serde_json::from_slice(&json_bytes).expect("uploaded JSON should be valid");
     let expected_json = serde_json::from_str(expected_json).expect("expected JSON does not parse");
-    find_json_mismatch(&expected_json, &actual_json)
-        .expect("uploaded JSON did not match expected JSON");
+
+    if let Err(e) = find_json_mismatch(&expected_json, &actual_json) {
+        panic!("{}", e);
+    }
 
     // 32-bit little-endian integer of length of crate file.
     let crate_sz = read_le_u32(&mut f).expect("read crate length");