Avoid parsing flags in test init

[HNO3Miracle]

Summary:

• Register the custom -save test flag without calling flag.Parse() in init.
• This avoids failing Go test binaries before Go registers its own -test.* flags.

Testing:

• GOPATH=/tmp/upstream-gopath GO111MODULE=off go test github.com/fogleman/gg -run '^$'

Note:

• Full tests still depend on GOPATH dependencies and image hashes outside this change.