Set up mutation tests

[thewilkybarkid]

Configures and runs Stryker, a mutation tester.

[erkannt]

Why are these additional explicit exports required. Doesn't the makefile set these up at the top?

[thewilkybarkid]

It forces TARGET to be dev for test:mutation (ie TARGET=prod make test:mutation continues to work as TARGET=prod is ignored).

[giorgiosironi]

what's the time or mutation/tests number budget here? In theory it could run forever but there must be a limit in place.

[thewilkybarkid]

https://github.com/stryker-mutator/stryker/tree/master/packages/core#timeoutms-number

[giorgiosironi]

What I mean is how to make it generate more mutations, but the code may be too simple for that. Adding a couple of operations in src/app.ts raises the number of mutants:

Ran 1.60 tests per mutant on average.
----------|---------|----------|-----------|------------|----------|---------|
File      | % score | # killed | # timeout | # survived | # no cov | # error |
----------|---------|----------|-----------|------------|----------|---------|
All files |   25.00 |        1 |         0 |          3 |        0 |       1 |
 app.ts   |   25.00 |        1 |         0 |          3 |        0 |       1 |
----------|---------|----------|-----------|------------|----------|---------|
14:03:00 (17) ERROR MutationTestReportCalculator Final mutation score 25.00 under breaking threshold 100, setting exit code to 
1 (failure).

[thewilkybarkid]

libero/article-store#167 shows more usage.

[giorgiosironi]

Explored whether to set high thinking that the 80% default wouldn't be matching. Doesn't change anything.

[erkannt]

lgtm