GdbTest::testStackSwitchThread: adjust shifted line number

e2911be2 introduced
GdbTest::testStackSwitchThread() with the code breakpoint on the
(one-based) line 39. The same commit added the file debugeethreads.cpp
with the code `    usleep(600);` on the line 39.

2.5 years later 456b2f82 inserted an
include line close to the top of debugeethreads.cpp, which shifted the
39th line down by one. The breakpoint line number in
testStackSwitchThread() was never adjusted accordingly and the debugee
was paused before the two sleep calls from then on. Increment the
breakpoint line number to restore the original intention and robustness
of this test function.

This lack of sleep most likely prompted
cfdaa127 to make the thread count check
in testStackSwitchThread() less strict. Revert this misguided test fix.

The less strict QVERIFY() check is also less informative than the
original QCOMPARE() check. This prompted
519637dc to debug-print the actual
stackModel->rowCount(). Remove this now-useless output as well.

Fortunately, neither the relaxing of the thread count check nor the
debug-printing of the actual stackModel->rowCount() have been
transferred to LldbTest, so nothing to revert there.

Hopefully this commit will fix frequent GdbTest::testStackSwitchThread()
failures on the FreeBSD CI server.