report_time - The Rust Unstable Book

# `report-time`

The tracking issue for this feature is: #64888

The `report-time` feature adds a possibility to report execution time of the tests generated via `libtest`.

This is unstable feature, so you have to provide `-Zunstable-options` to get this feature working.

Sample usage command:

    ./test_executable -Zunstable-options --report-time

Available options:

    --report-time [plain|colored]
                    Show execution time of each test. Awailable values:
                    plain = do not colorize the execution time (default);
                    colored = colorize output according to the `color`
                    parameter value;
                    Threshold values for colorized output can be
                    configured via
                    `RUST_TEST_TIME_UNIT`, `RUST_TEST_TIME_INTEGRATION`
                    and
                    `RUST_TEST_TIME_DOCTEST` environment variables.
                    Expected format of environment variable is
                    `VARIABLE=WARN_TIME,CRITICAL_TIME`.
                    Not available for --format=terse
    --ensure-time
                    Treat excess of the test execution time limit as
                    error.
                    Threshold values for this option can be configured via
                    `RUST_TEST_TIME_UNIT`, `RUST_TEST_TIME_INTEGRATION`
                    and
                    `RUST_TEST_TIME_DOCTEST` environment variables.
                    Expected format of environment variable is
                    `VARIABLE=WARN_TIME,CRITICAL_TIME`.
                    `CRITICAL_TIME` here means the limit that should not be
                    exceeded by test.

Example of the environment variable format:

    RUST_TEST_TIME_UNIT=100,200

where 100 stands for warn time, and 200 stands for critical time.

## Examples

    cargo test --tests -- -Zunstable-options --report-time
        Finished dev [unoptimized + debuginfo] target(s) in 0.02s
         Running target/debug/deps/example-27fb188025bec02c

    running 3 tests
    test tests::unit_test_quick ... ok <0.000s>
    test tests::unit_test_warn ... ok <0.055s>
    test tests::unit_test_critical ... ok <0.110s>

    test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

         Running target/debug/deps/tests-cedb06f6526d15d9

    running 3 tests
    test unit_test_quick ... ok <0.000s>
    test unit_test_warn ... ok <0.550s>
    test unit_test_critical ... ok <1.100s>

    test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out