Don't set a variable and export it at the same time

Some shells do not permit the form "export VAR=value", instead requiring
the variable to be assigned before it is exported.  Most places already
do this, fix those that don't.

Signed-off-by: John Keeping <john@keeping.me.uk>
Allow users to override the test extension

Signed-off-by: John Keeping <john@keeping.me.uk>
Merge pull request #6 from johnkeeping/allow-extension-override

Allow extension override