[lldb/test] Fix TestPlatform*.py Windows failures (NFC)

This patch fixes the windows failures introduced by `addb5148`:
http://lab.llvm.org:8011/builders/lldb-x64-windows-ninja/builds/18671/steps/test/logs/stdio



This macro, used in the test to check the platform, was missing a `_`,
making the test behave like it was run from a UNIX platform.

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>