runtime and agent test improvements

[zackattack01]

runtime

• renames testOsqueryBinaryDirectory to testOsqueryBinary for clarity. this was being used as a full path, and our download helpers alter this path to add .exe but our global variable was not getting the same update, causing some confusing error messages. e.g. no binary at C:\Users\zack-\AppData\Local\Temp\3167415370\osqueryd when one does exist at C:\Users\zack-\AppData\Local\Temp\3167415370\osqueryd.exe
• adds the .exe extension to the variable for windows so those error messages stop
• the osqueryd.exe, and the parent temp dir was also never being cleaned up due to use of defer before os.Exit. this updates to use explicit removal calls for all error cases and before exiting
• i do not expect this to fix test flakiness (I think we have various bits of logic that know to add the exe when required). hope is that these will be less confusing to debug and will stop leaking old test artifacts

agent

• noticed while looking into this that there were also several osquery-runsimple directories left behind- looking at the agent reset_test.go we have the same defer vs os.Exit problem. this one does seem to add the .exe correctly so nothing needed there