rpcclient: remove the 'for' variable "tc"

[jjjike2021]

Change Description

Description of change / link to associated issue.

rpcclient/backend_version_test.go:62:3: The copy of the 'for' variable "tc" can be deleted (Go 1.22+) (copyloopvar)
                tc := tc
                ^
rpcclient/backend_version_test.go:99:3: The copy of the 'for' variable "tc" can be deleted (Go 1.22+) (copyloopvar)
                tc := tc
                ^
rpcclient/infrastructure_test.go:96:3: The copy of the 'for' variable "tc" can be deleted (Go 1.22+) (copyloopvar)

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