[Backtracing] Fix tests up for Windows.
We can't use `|| true` on Windows, so use `not` instead.
Also, on Windows, program names get downcased because `lit` uses
`os.path.normcase()`, so if we have program names in the output,
make sure the programs are already lower-case.
Fix up the stack overflow test to match the expected output.
Plus fix the macOS build to always build for macOS 26.0 or above.
rdar://101623384
