[iOS] Skip test suites that take too long to build on helix (#72849)
System.Text.RegularExpressions, Microsoft.Extensions.Logging.Generators.Roslyn4.0.Tests, and STJ source generator tests take too long to build on helix and time out before any tests are run. This is due in part to the amount of aot'd code, which ends up ballooning the app size to around 1GB. The solution is to enable aggressive trimming targets that are in tests.mobile.targets as this will shrink the app size considerably.
Skipping these tests for now.
Addresses https://github.com/dotnet/runtime/issues/72834
Addresses https://github.com/dotnet/runtime/issues/72836