[lldb][test] Force lldb-server use in fail_watchpoint_command.test (#236711)
Turns out that our Windows on Arm bot hasn't been testing lldb-server
this entire time. We set LLDB_USE_LLDB_SERVER, but to test it you have
to set LLDB_TEST_USE_LLDB_SERVER.
And I don't fancy setting that and waiting a week to find out what
breaks.
So we will update in the future bot but for now, make this test always
use lldb-server.
With in-process debug, you cannot set any watchpoints on Windows on Arm.
With lldb-server, you can set 1, at least you can on the bot's hardware.
Fixes #235985. Relates to #232235.