test: don't overwrite the actual install when testing

Currently, the test fails if the install prefix is set to /usr/local (or
some other root-requiring directory) or it overwrites any previous
install that may have been done for posterity. Instead, set DESTDIR so
that it installs into the build tree.

Change-Id: Icd9aacac8df29fc5cc271940ce20edadff4ec9ce
Merge topic 'fix-install-test' into next

ff18672e test: don't overwrite the actual install when testing