Fix tests with Go 1.18

[simonpasquier]

trafficstars

go test ./... failed with Go 1.18 because the server certificate and key used SHA1. To fix this, the server certificate/key (and certificate authority chain) have been copied from the github.com/prometheus/common repository.

Additionally one test failed because the error message returned by the stdlib has been updated in Go 1.18. The test has been modified to support both variants.

Signed-off-by: Simon Pasquier [email protected]

Closes #83