Skip batch-preserve-term-settings.exp tests that cannot work on Windows.
    
    This group of tests assume that the gdb "shell" command launches a
    POSIX-compliant shell supporting the PPID environment variable, which
    is used to get gdb's pid for killing it from a remote_exec shell.  But
    on Windows host "shell" launches cmd.exe, which doesn't have an
    equivalent query.
    
    2019-08-15  Sandra Loosemore  <sandra@codesourcery.com>
    
    	gdb/testsuite/
    	* gdb.base/batch-preserve-term-settings.exp
    	(test_terminal_settings_preserved_after_sigterm): Skip on Windows.