Disabled ``implicitConversionFromPackageObjectShouldBeInScope`` PC test

Disabling the test ``implicitConversionFromPackageObjectShouldBeInScope`` from
PresentationCompilerTest because it often fails on our Jenkins machine (and
that's just plain annoying), e.g.,
https://jenkins.scala-ide.org:8496/jenkins/job/parameterized-scala-ide/168/console.
This test has been flaky for at least a month now.

Re #1001588