libtest: Add --report-time flag to print test execution time

[jakoschiko]

Implements the flag --report-time to print the execution time of each executed (successful or failed) test.

Closes #46610

Example

cargo test -- --report-time produces the following output to stdout:

running 6 tests
test tests::ignore ... ignored
test tests::noop ... ok 0.000s
test tests::should_panic ... ok 0.000s
test tests::panic_after_10millis ... FAILED 0.010s
test tests::sleep_100millis ... ok 0.100s
test tests::sleep_10secs ... ok 10.001s

failures:

---- tests::panic_after_10millis stdout ----
thread 'tests::panic_after_10millis' panicked at 'foo', src\lib.rs:31:9


failures:
    tests::panic_after_10millis

test result: FAILED. 4 passed; 1 failed; 1 ignored; 0 measured; 0 filtered out

cargo test -- --report-time -Z unstable-options --format=json produces the following output to stdout:

{ "type": "suite", "event": "started", "test_count": 6 }
{ "type": "test", "event": "started", "name": "tests::ignore" }
{ "type": "test", "event": "started", "name": "tests::noop" }
{ "type": "test", "event": "started", "name": "tests::panic_after_10millis" }
{ "type": "test", "event": "started", "name": "tests::should_panic" }
{ "type": "test", "name": "tests::ignore", "event": "ignored" }
{ "type": "test", "event": "started", "name": "tests::sleep_100millis" }
{ "type": "test", "name": "tests::noop", "event": "ok", "exec_time": "0.000s" }
{ "type": "test", "event": "started", "name": "tests::sleep_10secs" }
{ "type": "test", "name": "tests::should_panic", "event": "ok", "exec_time": "0.000s" }
{ "type": "test", "name": "tests::panic_after_10millis", "event": "failed", "exec_time": "0.010s", "stdout": "thread 'tests::panic_after_10millis' panicked at 'foo', src\\lib.rs:31:9\n" }
{ "type": "test", "name": "tests::sleep_100millis", "event": "ok", "exec_time": "0.101s" }
{ "type": "test", "name": "tests::sleep_10secs", "event": "ok", "exec_time": "10.000s" }
{ "type": "suite", "event": "failed", "passed": 4, "failed": 1, "allowed_fail": 0, "ignored": 1, "measured": 0, "filtered_out": 0 }

cargo test -- --report-time --logfile foo.log produces the following logfile:

ignored tests::ignore
ok tests::noop 0.000s
ok tests::should_panic 0.000s
failed tests::panic_after_10millis 0.010s
ok tests::sleep_100millis 0.100s
ok tests::sleep_10secs 10.001s