Fix non-constant format string in call in tests

[itchyny]

Go 1.24 will add verification of non-constant format strings.
It runs vet on testing so it fails on the mackerel-client-go tests.
See golang/go#60529.

 $ gotip test ./...                      
# github.com/mackerelio/mackerel-client-go
# [github.com/mackerelio/mackerel-client-go]
./mackerel_test.go:129:12: non-constant format string in call to (*github.com/mackerelio/mackerel-client-go.Client).tracef
./mackerel_test.go:147:12: non-constant format string in call to (*github.com/mackerelio/mackerel-client-go.Client).tracef
./mackerel_test.go:165:12: non-constant format string in call to (*github.com/mackerelio/mackerel-client-go.Client).tracef
./mackerel_test.go:182:12: non-constant format string in call to (*github.com/mackerelio/mackerel-client-go.Client).tracef
FAIL    github.com/mackerelio/mackerel-client-go [build failed]
FAIL

[yohfee]

Thank you for reporting!