👩‍🌾 Disable more tests known to fail on macOS and Windows

[chapulina]

• Disabled several C4251 warnings about exposing dll interfaces. There are triggered by stl types like std::unique_ptr and can be safely ignored because we don't officially support mixing compilers.
• Disabled warnings coming from ign-msgs and ign-rendering. Once the warnings are handled upstream, we should remove the pragmas from here.
• Disabled tests that are known to fail, and are documented on Re-enable skipped tests for OSX and Windows #75
• Actually fixed some compiler warnings

[codecov]

Codecov Report

Merging #112 into ign-gui2 will not change coverage.
The diff coverage is 20.00%.

@@            Coverage Diff            @@
##           ign-gui2     #112   +/-   ##
=========================================
  Coverage     60.43%   60.43%           
=========================================
  Files            16       16           
  Lines          2108     2108           
=========================================
  Hits           1274     1274           
  Misses          834      834           

Impacted Files	Coverage Δ
include/ignition/gui/Plugin.hh	100.00% <ø> (ø)
src/ign.cc	37.83% <ø> (ø)
src/plugins/publisher/Publisher.cc	32.89% <ø> (ø)
src/plugins/scene3d/Scene3D.cc	12.86% <0.00%> (ø)
src/plugins/topic_echo/TopicEcho.cc	30.50% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e57b711...176c36e. Read the comment docs.

[JShep1]

LGTM, CI is happy and local codecheck passes!