Add test for errors.Is()

[vtopc]

# go test -p 1 ./errors/... -race -timeout 1m --count=1

--- FAIL: TestIs (0.00s)
    --- FAIL: TestIs/holster_wrap (0.00s)
        go113_test.go:35:
            	Error Trace:	go113_test.go:35
            	Error:      	Should be true
            	Test:       	TestIs/holster_wrap
        go113_test.go:36:
            	Error Trace:	go113_test.go:36
            	Error:      	Target error should be in err chain:
            	            	expected: "wrapped"
            	            	in chain: "some reason: wrapped"
            	Test:       	TestIs/holster_wrap
FAIL
FAIL	github.com/mailgun/holster/v4/errors	5.011s