build: drop -i flag from test target
the -i flag has been deprecated in go 1.16. See https://golang.org/doc/go1.16
The -i build flag
The -i flag accepted by go build, go install, and go test is now deprecated. The -i flag instructs the go command to install packages imported by packages named on the command line. Since the build cache was introduced in Go 1.10, the -i flag no longer has a significant effect on build times, and it causes errors when the install directory is not writable.