Race on waddrmgr.newSecretKey during tests

[jrick]

==================
WARNING: DATA RACE
Write at 0x0000009259b8 by goroutine 9:
  github.com/btcsuite/btcwallet/waddrmgr.TstRunWithReplacedNewSecretKey()
      /home/travis/gopath/src/github.com/btcsuite/btcwallet/waddrmgr/internal_test.go:38 +0x79
  github.com/btcsuite/btcwallet/waddrmgr_test.testChangePassphrase()
      /home/travis/gopath/src/github.com/btcsuite/btcwallet/waddrmgr/manager_test.go:994 +0xcd
  github.com/btcsuite/btcwallet/waddrmgr_test.testManagerAPI()
      /home/travis/gopath/src/github.com/btcsuite/btcwallet/waddrmgr/manager_test.go:1350 +0x90
  github.com/btcsuite/btcwallet/waddrmgr_test.TestManager()
      /home/travis/gopath/src/github.com/btcsuite/btcwallet/waddrmgr/manager_test.go:1743 +0x8a6
  testing.tRunner()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:610 +0xc9

Previous read at 0x0000009259b8 by goroutine 10:
  [failed to restore the stack]

Goroutine 9 (running) created at:
  testing.(*T).Run()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:646 +0x52f
  testing.RunTests.func1()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:793 +0xb9
  testing.tRunner()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:610 +0xc9
  testing.RunTests()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:799 +0x4ba
  testing.(*M).Run()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:743 +0x12f
  main.main()
      github.com/btcsuite/btcwallet/waddrmgr/_test/_testmain.go:66 +0x1b8

Goroutine 10 (running) created at:
  testing.(*T).Run()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:646 +0x52f
  testing.RunTests.func1()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:793 +0xb9
  testing.tRunner()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:610 +0xc9
  testing.RunTests()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:799 +0x4ba
  testing.(*M).Run()
      /home/travis/.gimme/versions/go1.7.3.linux.amd64/src/testing/testing.go:743 +0x12f
  main.main()
      github.com/btcsuite/btcwallet/waddrmgr/_test/_testmain.go:66 +0x1b8
==================
FAIL	github.com/btcsuite/btcwallet/waddrmgr	2.118s

This race only exists because a function was made into a function variable so it could be modified by tests to change the function's behavior. The test is wrong, and must either be removed or fixed, but normal program operation is not affected because the variable is never written outside of tests.

Possible decisions:

• Make the function variable a normal function again and remove the test
• Rewrite the test to be correct, however this will require not just changes to the test code, but also modifications to the actual waddrmgr code.