Auto merge of #11028 - ehuss:test-errors, r=weihanglo

Auto merge of #11028 - ehuss:test-errors, r=weihanglo

Rework test error handling

This reworks how errors are handled when running tests and benchmarks. There were some cases where Cargo was eating the actual error and not displaying it. For example, if a test process fails to launch, it only displayed the `could not execute process` message, but didn't explain why it failed to execute. This fixes it to ensure that the full error chain is displayed.

This also tries to simplify how the errors are handled, and makes them more uniform across `test` and `bench`, and with doctests.

This also changes the `--no-fail-fast` behavior to report errors as they happen instead of grouped at the end (and prints a summary at the end). This helps to make it clearer when a nonstandard error happens. For example, before:

```
     Running tests/t1.rs (target/debug/deps/t1-bb449dfa37379ba1)

running 1 test
     Running tests/t2.rs (target/debug/deps/t2-1770ae8367bc97ce)

running 1 test
test bar ... FAILED

failures:

---- bar stdout ----
thread 'bar' panicked at 'y', tests/t2.rs:3:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
    bar

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed.

Caused by:
  process didn't exit successfully: `/Users/eric/Temp/z12/target/debug/deps/t1-bb449dfa37379ba1` (signal: 11, SIGSEGV: invalid memory reference)
  process didn't exit successfully: `/Users/eric/Temp/z12/target/debug/deps/t2-1770ae8367bc97ce` (exit status: 101)
```

and the changes to that are:

```diff
`@@` -1,6 +1,10 `@@`
      Running tests/t1.rs (target/debug/deps/t1-bb449dfa37379ba1)

 running 1 test
+error: test failed, to rerun pass `--test t1`
+
+Caused by:
+  process didn't exit successfully: `/Users/eric/Temp/z12/target/debug/deps/t1-bb449dfa37379ba1` (signal: 11, SIGSEGV: invalid memory reference)
      Running tests/t2.rs (target/debug/deps/t2-1770ae8367bc97ce)

 running 1 test
`@@` -18,8 +22,7 `@@`

 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

-error: test failed.
-
-Caused by:
-  process didn't exit successfully: `/Users/eric/Temp/z12/target/debug/deps/t1-bb449dfa37379ba1` (signal: 11, SIGSEGV: invalid memory reference)
-  process didn't exit successfully: `/Users/eric/Temp/z12/target/debug/deps/t2-1770ae8367bc97ce` (exit status: 101)
+error: test failed, to rerun pass `--test t2`
+error: 2 targets failed:
+    `--test t1`
+    `--test t2`
```

In the first example, when it says `Running tests/t1.rs`, there is no error message displayed until after all the tests finish, and that error message is not associated with the original test. This also includes the "to rerun" hint with `--no-fail-fast`.