Allow per-TestConfiguration ICoverageConfiguration

At the moment, coverage configuration (from ICoverageConfiguration) is defined globally:

export interface IConfigurationWithGlobalOptions {
    /**
     * Test configurations to run.
     */
    tests: TestConfiguration[];
    /**
     * Configuration used for handling coverage.
     */
    coverage?: ICoverageConfiguration;
}

This is unfortunately a bit limiting, as one cannot collect coverage separately, e.g., for integration vs. unit tests defined in two separate TestConfigurations. (Of course, this would also require that https://github.com/microsoft/vscode-test-cli/issues/38 be fixed; it both configurations write to the same output directory, this won't work.)

Start by tracing IConfigurationWithGlobalOptions, its tests array, and the existing ICoverageConfiguration handling. Determine how coverage can be associated with each TestConfiguration, while reviewing issue #38 because shared output directories currently block separate collection. Done means per-test coverage configuration is supported without relying on one global coverage setting.