Rather than regenerating it over and over we generate the test
platform as check_DATA and refer to it from the individual tests.
This cuts down "time make check -j8" from 1:25 to 1:06 in user time on
my machine, which is nice in itself, but it is also a stepping stone
for splitting out our large test-scripts into smaller ones where it
will make even more difference.
For the installed-tests case we fall back to creating the platform
once per test.
Closes: #2188
Approved by: alexlarsson
b5d86fe90b
