pkg/bisect: speed up tests
pkg/bisect: speed up tests

Creating new repos takes majority of test time.
Reuse them across tests.
With GOMAXPROCS=2 (CI) this speeds up tests from 33 sec to 7 secs.

xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller