Add ability to test_logs against kwargs.
Currently, you cannot use `@test_logs` to test a log message's keyword
arguments. This extends `@test_logs` to let you do this.
Additionally, while I was here, I also changed `@test_logs` itself to
let you specify _patterns_ using keyword args / NamedTuples, so that you
can more easily specify parts of the LogRecord that come at the end
(like the `kwargs`).
Here is an example of a log message with kwargs:
```julia
julia> @info "hi" x=2
┌ Info: hi
└ x = 2
```
Which produces this LogRecord:
```julia
LogRecord(Info, "hi", Main, Symbol("REPL[5]"), :Main_c95d07ae, "REPL[5]", 2, Base.Pairs(:x => 2))
```
After this PR, you can test for this kind of log message by matching the
`:kwargs` field to `@test_logs`:
```julia
@test_logs (:info, "hi", Test.Ignored(), Test.Ignored(), Test.Ignored(), Test.Ignored(), Test.Ignored(), (;y=3)) @info("hi", x=2)
```
But obviously the above is way verbose, so as I described in the second
paragraph, this PR also adds the following shorter syntax:
```julia
@test_logs (level=:info, message="hi", kwargs=(;x=2)) @info("hi", x=2)
```
Also updated the docstring.
