Convert t.Fatal to t.Error and return to follow vet recommendation

[aidan-kwon]

Proposed changes

make test fails with the following errors when the machine use golang v1.16 or higher.

blockchain/blockchain_test.go:1042:6: call to (*T).Fatalf from a non-test goroutine

From go1.16, Vet warns the use of t.Fatal in goroutines during the test.
And, the warning stops the further process of make test.
So, all t.Fatal in goroutines in test files are converted to t.Error and return as golang release note suggested.

Reference: Vet section in go1.16 Release note

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[kjhman21]

LGTM