Test: Replace t.error/fatal with assert/request in [/confchange]

Refactor: Replacing t.Error/t.Fatal with assert/require

This commit tries to eliminate boilerplate code in the testing suite by replacing occurrences of t.Error and t.Fatal with assertions from the assert and require packages.

Changes Made:

Replaced instances of t.Error with assert
Replaced instances of t.Fatal with require

Affected Files:

[confchange/quick_test.go]
[confchange/restore_test.go]

Part of: #146

etcd-io / raft

Test: Replace t.error/fatal with assert/request in [/confchange] #186