tests: TestMagicNames: add warmup rounds
tests: TestMagicNames: add warmup rounds

Chasing a bug that seems to have nothing to do
with magic names, as it already triggers during
warmup:

--- FAIL: TestMagicNames (0.00s)
    matrix_test.go:773: Testing n="warmup1"
    matrix_test.go:773: Testing n="warmup2"
    matrix_test.go:820: no such file or directory

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs