fix: fix formatting of Karma stack traces

[vikerman]

Make the paths(with and without sourcemaps) relative to the project root so that the file locations are linkified in editors like VS Code.

Tested in angular/angular using VS Code.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Karma error stacks used to look like this.

    Expected true to be false.
        at UserContext.eval (http://concatjsangular/packages/core/test/application_init_spec.ts:19:31 <- http://concatjangular/packages/core/test/application_init_spec.js:27:41)
        at TestBedViewEngine.execute (http://concatjsangular/packages/core/testing/src/test_bed.ts:486:14 <- http://concatjsangular/packages/core/testing/src/test_bed.js:372:23)
        at UserContext.eval (http://concatjsangular/packages/core/testing/src/test_bed.ts:690:39 <- http://concatjsangular/packages/core/testing/src/test_bed.js:542:49)
        at ZoneDelegate.invoke (http://concatjsnode_modules/zone.js/dist/zone.js:388:26)

What is the new behavior?

Karma error stacks look like this now and clickable from VS Code terminal.

    Expected true to be false.
        at UserContext.eval (packages/core/test/application_init_spec.ts:19:31 <- angular/packages/core/test/application_init_spec.js:27:41)
        at TestBedViewEngine.execute (packages/core/testing/src/test_bed.ts:486:14 <- angular/packages/core/testing/src/test_bed.js:372:23)
        at UserContext.eval (packages/core/testing/src/test_bed.ts:690:39 <- angular/packages/core/testing/src/test_bed.js:542:49)
        at ZoneDelegate.invoke (node_modules/zone.js/dist/zone.js:388:26)

Does this PR introduce a breaking change?

• Yes
• No

[filipesilva]

Is there a reasonable way we could test this is working as intended?