Disable bug-18026.exe when running tests in the CI environment

The test performed in bug-18026.exe can sometimes fail if the hazard
table gets full. Disabling it in CI testing avoids unwanted noise.

See https://bugzilla.xamarin.com/show_bug.cgi?id=19235
Merge pull request #1000 from ranma42/disable-18026-on-wrench

Disable bug-18026.exe when running tests in the CI environment