Our exeter tests make use of exetool, a tool that's part of the exeter
tree. Both Yumei and I have seen cases where exetool is missing, despite
it being from an external tree we shouldn't be touching.
I haven't pinned down the exact circumstances in which this happens, but I
think this is occurring because the way the Makefile refers to it can make
it look like an intermediate target that make may delete when interrupted.
Mark it as .PRECIOUS to prevent that.
Signed-off-by: David Gibson