Improve selectivity of UsbDrive tests

 * Mark additional tests with RequiresUSB
 * Fail rather than crash when insufficient drives are mounted
 * Ignore some tests that are based on incorrect assumptions
Merge pull request #153 from neilmayhew/topic/usb-tests

Improve selectivity of UsbDrive tests
WS-273 "Help" link not visible in Linux

With Linux, the Help link does not move as the
screen size is adjusted.  If the original location
specified is not visible on the screen, the link
does not appear.  The table layout panel next to
this one containing the sort results adjusts
correctly. This change makes the parameters
associated with the two panels identical.  The
link now adjusts relative to the panel size in
both windows and linux.
Merge pull request #154 from sillsdev/WS273

WS-273 "Help" link not visible in Linux
Fix sizes of internal controls of AcquireImageControl

A Mono layout bug truncates the size of controls anchored right and
left inside AcquireImageControl.  This provides a workaround to fix
https://jira.sil.org/browse/BL-778 "[Linux] Image Toolbox only
displays 2 columns of AoR images (Windows does 3)".  The bug would
probably affect WeSay as well as Bloom.
Merge pull request #155 from sillsdev/Fix-BL-778

Fix sizes of internal controls of AcquireImageControl