TestCov: remove tmpnam (minor)

Following warning appears on Linux:

```
TestCov.cpp.o: in function `TestCov::read_covfile(TestCov::ScenarioType)':
TestCov.cpp:(.text+0xd104): warning: the use of `tmpnam' is dangerous, better use `mkstemp'
```

and on MacOS:

```
Due to security concerns inherent in the design of tmpnam(3), it is highly recommended that you use mkstemp(3) instead.
```