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
(cherry picked from commit 02fbfe2347e4d83de6f84fc0b5cb6e44f5bec19e)
Merge pull request #348 from dotta/backport/issue/flaky-presentation-compiler-test-1001588

[backport] Disabled ``implicitConversionFromPackageObjectShouldBeInScope`` PC test
Merge remote-tracking branch 'upstream/release/scala-ide-3.0.x' into release/scala-ide-3.0.x-juno