[lldb] Skip target variable tests on Darwin because of chained fixups

When targeting macOS Ventura, ld64 will use authenticated fixups for
x86_64 as well as arm64 (where that has always been the case). This
results in test failures when using an Xcode 14 toolchain on an Intel
mac running macOS Ventura:

  Failed Tests (3):
    lldb-api :: commands/target/basic/TestTargetCommand.py
    lldb-api :: lang/c/global_variables/TestGlobalVariables.py
    lldb-api :: lang/cpp/char8_t/TestCxxChar8_t.py

Rather than trying to come up with a sophisticated decorator based off
the deployment target, I marked them all as skipped with a comment
explaining why.

Differential revision: https://reviews.llvm.org/D131741