Auto merge of #64873 - popzxc:prettify-test-time, r=wesleywiser

Auto merge of #64873 - popzxc:prettify-test-time, r=wesleywiser

Enhance report-time option

## Short overview

This PR is a follow-up to a previously closed #64714 PR.

## Changes introduced by this PR

* `libtest` now retrieves the type of the test within `TestDesc` (available types are: `UnitTest`, `IntegrationTest`, `DocTest`, `Unknown`).
* `--report-time` subcommand of the `libtest` now supports colored output (disabled by default).
* Colorized output depends on the threshold values. Default values (proposed by @wesleywiser):
  - For unit-tests: 50ms warn/100ms critical,
  - For integration-tests: 500ms warn/1000ms critical,
  - For doctests: same as for integration tests,
  - For unknown tests: `TEST_WARN_TIMEOUT_S` warn/ `TEST_WARN_TIMEOUT_S * 2` critical (it will only applied single-threaded mode, because otherwise test will be interrupted after reaching `TEST_WARN_TIMEOUT_S`).
  - These values can be overrided by setting environment variables (since those thresholds are somewhat constant for every project, it's more flexible to use environment variables than command line arguments).
* New optional flag `--ensure-test-time` for `libtest`. With this flag applied, exectuion time limit excesss will cause test failure.

## What have not been done

There was a comment that it would be nice to have an entry in the Cargo book about it.

However, changes introduced by this PR (and #64663 in which `report-time` flag was added) aren't related directly to `cargo`, it's more about `libtest` itself.
I'm considering that [The Unstable Book](https://doc.rust-lang.org/unstable-book/) is more appropriate place, but not sure if I'm right (and if so, how exactly it should be described).

As one possible option, this PR may be merged without denoting it in the documentation, and in the next PR adding support of this feature to the `cargo` itself, I'll add a note in the Cargo book.

## Scope of this PR

Logical scope of this PR is `libtest` only. However, to get test types, I had to modify also `libsyntax_ext` and `librustdoc` for them to provide information about test type.

## Rationale

Rationale for colored output was submitted in #64714

Providing the information about kind of test was also proposed in #64714, and as an additional benefit this information may be useful for the tools using `libtest` (e.g. `cargo`).

Adding flag to treat time limits excess seems logical to me, so projects that do care about test execution time won't have to invent a wheel.

## Backward compatibility

All the changes are completely backward compatible.

## Demo

![rustc_enhanced_time](https://user-images.githubusercontent.com/12111581/65818381-c04f6800-e219-11e9-9875-322463abe24f.gif)

r? @wesleywiser