misc/ios: always kill lldb process when it hangs

misc/ios: always kill lldb process when it hangs

The lldb child process is killed if a test runs too long. Also
kill it when the setup times out (and is retried).

Might help with builder flakes where all 5 attempts to start up
lldb fail even though the tests before and after the timeouts
succeed. For example:

...
ok  	vendor/golang_org/x/net/route	37.797s
lldb setup error: command timeout (lldb start for 17s)
start timeout, trying again
lldb setup error: command timeout (lldb start for 17s)
start timeout, trying again
lldb setup error: command timeout (lldb start for 17s)
start timeout, trying again
lldb setup error: command timeout (lldb start for 17s)
start timeout, trying again
lldb setup error: command timeout (lldb start for 17s)
go_darwin_arm_exec: failed to start test harness (retry attempted)
FAIL	vendor/golang_org/x/text/transform	115.185s
ok  	vendor/golang_org/x/text/unicode/norm	122.773s
...

Change-Id: I6638860522896491dccfa12f1e520c0f23df6d66
Reviewed-on: https://go-review.googlesource.com/67791
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>