Testing TestTools

Unit tests may be run using any of the following methods:

from the shell using the Make target
```
$ make test
```
from the Julia REPL
- without collection of coverage data
```
import Pkg; Pkg.test("TestTools")
```
- with collection of coverage data
```
import Pkg; Pkg.test("TestTools"; coverage=true)
```
from the shell using jltest
```
$ jltest -W test/runtests.jl
```
When using jltest to run the unit tests, note that the -W or --no-wrapper option is required for the tests that check the output failing tests to work correctly.

Note. Use this method to test the outcome of the unit tests when they are run by the julia-actions/julia-runtest GitHub Action.