test_runner: add coverage support with watch mode.

[pulkit-30]

What is the problem this feature will solve?

Currently, we can't get coverage report along with watch flag.

code:

import test from "node:test";

test("foo", () => {});

run cmd ./node/node --test --watch --experimental-test-coverage

output:

pulkitgupta@Pulkits-MacBook-Air jest-2 % ../node/node --test --watch --experimental-test-coverage
✔ foo (0.867791ms)
^C%                                                                                                                                                       
pulkitgupta@Pulkits-MacBook-Air jest-2 % 

no coverage report.

What is the feature you are proposing to solve the problem?

To show test coverage with watch mode, (something similar what jest does).

output:

pulkitgupta@Pulkits-MacBook-Air jest-2 % ../node/node --test --watch --experimental-test-coverage  
✔ foo (1.044042ms)
ℹ start of coverage report
ℹ ---------------------------------------------------------------
ℹ file           | line % | branch % | funcs % | uncovered lines
ℹ ---------------------------------------------------------------
ℹ index.test.mjs | 100.00 |   100.00 |  100.00 | 
ℹ ---------------------------------------------------------------
ℹ all files      | 100.00 |   100.00 |  100.00 |
ℹ ---------------------------------------------------------------
ℹ end of coverage report
^C%  

What alternatives have you considered?

No response

[MoLow]

what would be the trigger/point in time for the coverage report te be calculated?

[pulkit-30]

what would be the trigger/point in time for the coverage report te be calculated?

once all tests have completed.

I expected a similar behavior when I ran node --watch --experimental-test-coverage index.test.mjs.

[MoLow]

once all tests have completed.

they never complete, thats the point