chore(test): start fixing test types

[cartant]

Description:

At the moment, the test script runs the specs using mocha and uses ts-node/register to transpile the TypeScript. However, the configuration does not enable type checking and there are many, many errors.

This PR adds some temporary changes to the infrastructure to facilitate the incremental fixing of these type errors. Specifically it:

• Creates a temporary spec/tsconfig.check.json file.
• Specifies an include in said file so that files can be incrementally checked and fixed.
• Creates a temporary script named test_check in the package.json to run a no-emit tsc build that uses spec/tsconfig.check.json.
• Includes npm run test_check in the .travis.yml configuration file so that the CI build runs it.

With these changes it's possible to incrementally fix specific spec files - by adding them to the includes property in the spec/tsconfig.check.json file. This PR includes fixes for several files. However, fixes are still outstanding for all of the spec files in:

• spec/observables
• spec/operators
• spec/schedulers
• spec/subjects

The intention is for PRs to include individual commits for specific files. For example, add ./operators/buffer-spec.ts to the include property, fix any type errors, then include those fixes in a commit.

When all -spec.ts files pass, the temporary scripts can be removed from the package.json and the .travis.yml files and the spec/tsconfig.check.json file can be deleted.

At that stage, the TS_NODE_TYPE_CHECK environment variable can be added to the test script so that ts-node will throw an error if a TypeScript error is introduced into the tests.

Related issue (if exists): #3411

[coveralls]

Coverage remained the same at 96.833% when pulling da7bf69 on cartant:issue-3411 into dc66731 on ReactiveX:master.

[benlesh]

Thanks, @cartant! This is a solid add.