Rewrite "testPerm" helper function used for testing of internal
runtime code. It's been renamed to "unitTest" and provides API that
is extensible in the future by accepting optional "UnitTestOptions"
argument. "test" helper was also removed and replaced by
overloaded version of "unitTest" that takes only function argument.
"UnitTestOptions" currently supports "perms" and "skip"
options, where former works exactly as first argument to "testPerm"
did, while the latter allows to conditionally skip tests.
For some reason, the unit tests for Deno.remove() were not being imported to
unit_tests.ts and, consequently, not being executed. Thus, I imported them,
refactored some existent ones and wrote new ones for the symlink removal case.
Since the creation of a symlink is not implemented for Windows yet, assertions
that consider this state were added when the tests are executed in this OS.
