[lldb/test] Fix app_specific_backtrace_crashlog.test (NFC)

This patch changes app_specific_backtrace_crashlog.test's crashlog file
extension from `ips` to `txt. This should prevent the test from opening
Console.app when being run.

This should also fix a test failure caused by missing symbols.

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