Don't change britney.conf from the source tree and restore it back after the
tests; this is prone to leave a broken config behind if a test gets
interrupted. Instead, copy the file to our temp dir, and let tests hack on it
there.
(Introduced in lp:~canonical-ci-engineering/britney/boottesting-support)
Extend read_sources to store a new AUTOPKGTEST boolean flag, which is true if
the Testsuite: field exists and starts with "autopkgtest" (this covers autodep8
cases like autopkgtest-pkg-perl).
Extend TestData.add() to take a new testsuite argument which specifies the
source's Testsuite: field.
