Generating coverage statistics places *.cov files in .julia/packages directory

Generating coverage statistics using
```
TestReports.test("MyPackage"; coverage = true)
```
places *.cov files in my .julia/packages directory for all the dependencies of MyPackage.  These files accumulate over time, which is undesirable.

Generating coverage statistics using Pkg,
```
Pkg.test(coverage = true)
```
does not cause this behavior.