build error on vcursor_impl tests on newer versions of go

[ajm188]

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Overview of the Issue

When running the go/vt/vtgate/vcursor_impl_test.go tests, I get the following on go1.15

Running tool: /usr/local/bin/go test -timeout 30s -coverprofile=/var/folders/38/3_q_szqn7zx66d0cgj0g_z1w0000gp/T/vscode-go0YkAyW/go-code-cover vitess.io/vitess/go/vt/vtgate -run ^(TestDestinationKeyspace|TestSetTarget|TestPlanPrefixKey|TestFirstSortedKeyspace)$

# vitess.io/vitess/go/vt/vtgate
/Users/amason/work/vitess/go/vt/vtgate/vcursor_impl_test.go:240:9: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
/Users/amason/work/vitess/go/vt/vtgate/vcursor_impl_test.go:280:9: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
FAIL	vitess.io/vitess/go/vt/vtgate [build failed]
FAIL

This also happens if I run go test -v ./go/vt/vtgate myself.

Reproduction Steps

Install go1.15 and go1.14:

❯ /usr/local/opt/go@1.15/bin/go test -v ./go/vt/vtgate
# vitess.io/vitess/go/vt/vtgate
go/vt/vtgate/vcursor_impl_test.go:240:9: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
go/vt/vtgate/vcursor_impl_test.go:280:9: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
FAIL    vitess.io/vitess/go/vt/vtgate [build failed]
FAIL
❯ /usr/local/opt/go@1.14/bin/go test -v ./go/vt/vtgate
ERROR: logging before flag.Parse: I0929 19:04:38.557277   31538 buffer.go:144] vtgate buffer not enabled.
ERROR: logging before flag.Parse: I0929 19:04:38.557461   31538 discoverygateway.go:122] loading tablets for cells: 
ERROR: logging before flag.Parse: I0929 19:04:38.557488   31538 gateway.go:100] Gateway waiting for serving tablets of types [] ...
ERROR: logging before flag.Parse: I0929 19:04:38.557512   31538 gateway.go:108] Waiting for tablets completed
ERROR: logging before flag.Parse: I0929 19:04:38.557523   31538 vtgate.go:85] Transaction mode: 'MULTI'
ERROR: logging before flag.Parse: I0929 19:04:38.558026   31538 vschema_manager.go:74] Received vschema update
ERROR: logging before flag.Parse: I0929 19:04:38.558196   31538 streamlog.go:155] Streaming logs from VTGate at /debug/querylog.
=== RUN   TestAutocommitUpdateSharded
--- PASS: TestAutocommitUpdateSharded (0.00s)
[rest omitted]