Add '--skiptests' option to skip tests

[jyoungyun]

Steps to reproduce

./build.sh --skiptests

Expected behavior

[TARGET     <] [ OK ] [00:00:45.538] Compile                                                    (Microsoft.DotNet.Host.Build.CompileTargets.Compile)
Skipping, Target Conditions not met: Test
[TARGET     >] [....] [00:00:45.538] Package                                                    (Microsoft.DotNet.Host.Build.PackageTargets.Package)