Fix failing unit tests by slowing them down

W3C has rate limiter and resolving of base JSONLD contexts
fails if the test cases are executed too fast. This commit
artificially slows down test case execution, so tests could
pass without any significant changes.

Another option to resolve this issue is to expose these
base contexts at another URL without rate limiting just for use
in tests, but it will be more complicated solution and a lot more
unstable as the project tests will depend on our server running
properly 24x7 indefinely in the future.