fate.sh: Allow setting other make flags for running tests

fate.sh: Allow setting other make flags for running tests

If makeopts_fate is set, these makeopts are used for running the
tests instead of the normal makeopts. If it isn't set, the normal
makeopts variable is used as before.

This is useful if remote testing on a lesser machine where a large
number of parallel jobs might be undesireable, while wanting to speed
up the build with many parallel processes.

Signed-off-by: Martin Storsjö <martin@martin.st>