feat(testlab): rework itSkippedOnTravis to support Jest framework

[bajtos]

Describe referenced Mocha types directly in testlab to avoid compile-time dependency on Mocha types, some of which are not compatible with Jest -- fix #2452

Contents of packages/testlab/dist/skip-travis.d.ts:

BEFORE

/// <reference types="mocha" />
export declare type TestCallbackRetval = void | PromiseLike<any>;
/**
 * Helper function for skipping tests on Travis env
 * @param expectation
 * @param callback
 */
export declare function itSkippedOnTravis(expectation: string, callback?: (this: Mocha.ITestCallbackContext, done: MochaDone) => TestCallbackRetval): void;

Notice /// <reference types="mocha" /> which is the root cause of the problem reported in #2452

AFTER

export interface TestFn {
    (this: TestContext): PromiseLike<unknown>;
    (this: TestContext, done: Function): void;
}
export interface TestContext {
    skip(): this;
    timeout(ms: number | string): this;
    retries(n: number): this;
    slow(ms: number): this;
    [index: string]: any;
}
/**
 * Helper function for skipping tests on Travis env
 * @param expectation
 * @param callback
 */
export declare function itSkippedOnTravis(expectation: string, callback?: TestFn): void;

Checklist

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• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

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