dotc.tests.tasty_dotc_reporting is flacky

[DarkDimius]

http://ci.d-d.me/job/dotty-benchmarks/383/consoleFull failed

[DarkDimius]

unpickling from tasty failed:

cannot display due to dotty.tools.dotc.core.Denotations$StaleSymbol: stale symbol; method report#12314 in class Reporter, defined in Period(1..4, run = 2), is referred to in run Period(1..1, run = 3)