chore: enforce that pop.SetNowFunc only can be called inside init()

[gaultier]

When the function is not called inside init(), it looks like this:

--- FAIL: Test_Create_Timestamps_With_NowFunc (0.00s)
panic: caller init() not found in call stack, reached end of call stack [recovered, repanicked]

goroutine 616 [running]:
testing.tRunner.func1.2({0x10142a0a0, 0x10159bfc0})
	/opt/homebrew/Cellar/go/1.25.4/libexec/src/testing/testing.go:1872 +0x2b4
testing.tRunner.func1()
	/opt/homebrew/Cellar/go/1.25.4/libexec/src/testing/testing.go:1875 +0x460
panic({0x10142a0a0?, 0x10159bfc0?})
	/opt/homebrew/Cellar/go/1.25.4/libexec/src/runtime/panic.go:783 +0x120
github.com/ory/pop/v6.ensuredCalledInsideInit()
	/Users/philippe.gaultier/company-code/pop/model.go:40 +0x200
github.com/ory/pop/v6.SetNowFunc(...)
	/Users/philippe.gaultier/company-code/pop/model.go:48
github.com/ory/pop/v6.Test_Create_Timestamps_With_NowFunc.func1(0xc0004dcd80)
	/Users/philippe.gaultier/company-code/pop/executors_test.go:2007 +0x198
github.com/ory/pop/v6.transaction.func1(0xc0004dcd80)
	/Users/philippe.gaultier/company-code/pop/pop_test.go:73 +0x3c
github.com/ory/pop/v6.(*Connection).Rollback(0xc0000aaae0, 0xc00004be50)
	/Users/philippe.gaultier/company-code/pop/connection.go:196 +0x124
github.com/ory/pop/v6.transaction(0xc0005efe98)
	/Users/philippe.gaultier/company-code/pop/pop_test.go:72 +0x68
github.com/ory/pop/v6.Test_Create_Timestamps_With_NowFunc(0xc0004c0700)
	/Users/philippe.gaultier/company-code/pop/executors_test.go:1997 +0xa4
testing.tRunner(0xc0004c0700, 0x101599500)
	/opt/homebrew/Cellar/go/1.25.4/libexec/src/testing/testing.go:1934 +0x168
created by testing.(*T).Run in goroutine 1
	/opt/homebrew/Cellar/go/1.25.4/libexec/src/testing/testing.go:1997 +0x6e4
FAIL	github.com/ory/pop/v6	1.494s

which is why Test_Create_Timestamps_With_NowFunc was removed in this PR.

Tested successfully in kratos/kratos-oss.