feat(warn slow): warn users for slow runs

[nicojs]

Warn users for slow runs because a relatively big chunk of their mutants being static mutants.

• Implement unit tests
• Decide how to warn when estimated time is 50%, but static mutants are also 50%

Fixes #3435

[nicojs]

@simondel what do you think about this message?

02:33:39 (4545) INFO InputFileResolver Found 113 of 1037 file(s) to be mutated.
02:33:40 (4545) INFO Instrumenter Instrumented 113 source file(s) with 3309 mutant(s)
02:33:40 (4545) INFO ConcurrencyTokenProvider Creating 1 checker process(es) and 1 test runner process(es).
02:33:41 (4545) INFO Sandbox Running build command "tsc -b" in "/home/nicojs/github/stryker-js/packages/core/.stryker-tmp/sandbox1282307".
02:34:00 (4545) INFO DryRunExecutor Starting initial test run (mocha test runner with "perTest" coverage analysis). This may take a while.
02:34:04 (4545) INFO DryRunExecutor Initial test run succeeded. Ran 693 tests in 4 seconds (net 3420 ms, overhead 591 ms).
02:34:04 (4545) WARN MutantTestPlanner Detected 255 static mutants (8% of total) that are estimated to take 85% of the time running the tests!
  You might want to enable "ignoreStatic" to ignore these static mutants for your next run. 
  For more information about static mutants visit: https://stryker-mutator.io/docs/mutation-testing-elements/static-mutants.
  (disable "warnings.slow" to ignore this warning)
Mutation testing  [                                                  ] 0% (elapsed: <1m, remaining: n/a) 0/3305 Mutants tested (0 survived, 0 timed out)