mock: update callString to format context.Context as pointer

[jmank88]

Summary

This PR proposes updating func callString in package mock to format context.Context arguments as their pointer value, in order to avoid a data race from formatting the internals as a string.

Changes

• changed func callString to format context.Context args as their pointer value
• added subtest Test_callString/context to validate

Motivation

This is an example race detected from the new test Test_callString/context, before including the fix:

Data Race

==================
WARNING: DATA RACE
Write at 0x00c000016470 by goroutine 10:
  sync/atomic.AddInt32()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/runtime/race_amd64.s:281 +0xb
  sync/atomic.AddInt32()
      <autogenerated>:1 +0x14
  context.(*cancelCtx).cancel()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/context/context.go:561 +0x2e7
  context.WithCancel.func1()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/context/context.go:237 +0x52
  github.com/stretchr/testify/mock.Test_callString.func1.2()
      /home/jordan/testify/mock/mock_test.go:1248 +0x99

Previous read at 0x00c000016470 by goroutine 9:
  reflect.Value.Int()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/reflect/value.go:1458 +0x544
  fmt.(*pp).printValue()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/fmt/print.go:792 +0x4a0
  fmt.(*pp).printValue()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/fmt/print.go:853 +0x1d1e
  fmt.(*pp).printValue()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/fmt/print.go:853 +0x1d1e
  fmt.(*pp).printValue()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/fmt/print.go:921 +0x130a
  fmt.(*pp).printArg()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/fmt/print.go:759 +0xb84
  fmt.(*pp).doPrintf()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/fmt/print.go:1074 +0x5cf
  fmt.Sprintf()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/fmt/print.go:239 +0x5c
  github.com/stretchr/testify/mock.callString()
      /home/jordan/testify/mock/mock.go:453 +0x4ae
  github.com/stretchr/testify/mock.Test_callString.func1()
      /home/jordan/testify/mock/mock_test.go:1253 +0x217
  testing.tRunner()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1743 +0x44

Goroutine 10 (running) created at:
  github.com/stretchr/testify/mock.Test_callString.func1()
      /home/jordan/testify/mock/mock_test.go:1245 +0x19c
  testing.tRunner()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1743 +0x44

Goroutine 9 (running) created at:
  testing.(*T).Run()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1743 +0x825
  github.com/stretchr/testify/mock.Test_callString()
      /home/jordan/testify/mock/mock_test.go:1238 +0x195
  testing.tRunner()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      /home/jordan/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1743 +0x44
==================