Don't crash after killing CTestFindJob

The m_suite might have been deleted at that point already, so it's
not safe to try to output it's name anymore. Change the logic to
instead unset the m_suite member and don't try to access it once
unset anymore.

Fixes:
```
Thread 1 "kdevelop" received signal SIGSEGV, Segmentation fault.
0x00007fff3949a9b0 in CTestFindJob::updateReady (this=0x55555a60e770, document=..., context=...) at /home/milian/projects/kde/src/kdevelop/plugins/cmake/testing/ctestfindjob.cpp:64
64              qCDebug(CMAKE) << "Cannot add test suite" << m_suite->name() << ": this job has been killed.";
(gdb) p m_suite
$1 = (CTestSuite *) 0x55555a6057b0
```