Temporarily disable linux-fatal-backtrace.swift.

[bob-wilson]

This test is failing and blocking PR testing for Linux. I am disabling it
while we sort out the problem.

rdar://problem/33718631

[bob-wilson]

@swift-ci please smoke test and merge

[bob-wilson]

Now the PR test failed for some other reason related to yesterday's temporary change to remove lldb from PR testing:

lit.py: /home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/branch-master/swift-integration-tests/lit.cfg:170: note: testing using 'lldb': /home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/branch-master/buildbot_linux/none-swift_package_sandbox_linux-x86_64/usr/bin/lldb
lit.py: /home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/branch-master/swift-integration-tests/lit.cfg:180: fatal: lldb does not exist!

I think we might be better off to just get LLDB working again.

[bob-wilson]

Mishal has a solution for that other problem. I'm going to go ahead and merge this.

[bob-wilson]

PR to reenable the test: #12928