* Add ZIO.assert

The `assert` combinator is useful for introducing mock-style assertions in your tests.

Example use case:
* you have a `Command` type method which takes some parameters
* you want to check if it was called with expected arguments
* you do not want to introduce heavy libraries like Mockito

```scala
def send(recipient: String, content: String): ZIO[R, InvalidEmailAddress, Unit]

// for test purposes be implemented as follows
def send(recipient: String, content: String) =
  UIO.unit
    .assert(recipient == "[email protected]", s"""Invalid recipient $recipient, expected "[email protected]".""")
    .assert(content == "lorem ipsum dolor sit amet", s"""Invalid content $content, expected "lorem ipsum dolor sit amet".""")
```

@adamgfraser We might also have some special handling for `AssertionError` in `zio-test`, to replace long stack trace (for failed fiber) with a shorter one. What do you think?

* CS fix

* Code review

* Code review #2

* Die with zio.AssertionFailure

* Disable trace reporting

* Reduce PR scope

* Migrate all tests to ZIO Test #1647 - ExecutorSpec

* Migrate all tests to ZIO Test #1647 - ExecutorSpec, patch #2

* Migrate all tests to ZIO Test #1647 - ExecutorSpec, patch #3

* Migrate all tests to ZIO Test #1647 - ExecutorSpec, patch #4

* #1647-ExecutorSpec

* Migrate all tests to ZIO Test #1647 - ExecutorSpec, patch #5

* Migrate all tests to ZIO Test #1647 - ExecutorSpec, patch #6

* Migrate all tests to ZIO Test #1647 - ExecutorSpec, patch #7

* Composable mocks

* ZIO Mock replace implicits with ZLayer

* Examples & docs

* Fix 2.11 warnings

* Fix cyclic reference issue on 2.11

* Code review

* Code review #2

* Refactor

* mdoc:silent

Before this change, the message looked like the one below, which was difficult to interpret:
Interrupted by fibers: #S, zio#4, zio#3, #t, zio#5, zio#2, #(, #e, #)