chore: add test for embedded IsZero() implementer

[smlx]

I had a failing test in a project I'm working on that I tracked down to what I think is a bug in https://github.com/alecthomas/repr. However, it was exposed by my use of assert and I managed to find a simple reproducer test case that exposes the bug so I have sent the PR here.

Basically, it looks like the change around IsZero() in v0.5.x of repr causes a panic in some cases. I see that you have fixed one such case in v0.5.2 of repr, but that doesn't fix the case that I found in this PR: when comparing a struct with a pointer member that is nil, but has an embedded type that implements IsZero() (such as time.Time), it will panic.

Reverting to repr v0.4.0 allows this test case to pass.

Please let me know if I can provide more info.

Here's the panic you get running this PR:

--- FAIL: TestEqual (0.00s)
    --- FAIL: TestEqual/Embedded_time.Time (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x54af89]

goroutine 15 [running]:
testing.tRunner.func1.2({0x5787e0, 0x6f3b30})
	/usr/local/go/src/testing/testing.go:1872 +0x237
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1875 +0x35b
panic({0x5787e0?, 0x6f3b30?})
	/usr/local/go/src/runtime/panic.go:783 +0x132
github.com/alecthomas/assert/v2.(*MyTime).IsZero(0x6f47b0?)
	<autogenerated>:1 +0x9
github.com/alecthomas/repr.(*Printer).reprValue(0xc000013180, 0xc0000e7c78, {0x57d180, 0x0, 0x19}, {0x0, 0x0}, 0x1, 0x0)
	/home/scott/go/pkg/mod/github.com/alecthomas/[email protected]/repr.go:302 +0x1a66
github.com/alecthomas/repr.(*Printer).Print(0xc000013180, {0xc0000e7d18?, 0x1, 0x1?})
	/home/scott/go/pkg/mod/github.com/alecthomas/[email protected]/repr.go:175 +0xf5
github.com/alecthomas/repr.String({0x57d180, 0x0}, {0xc0000780f0, 0x1, 0x6c8b28?})
	/home/scott/go/pkg/mod/github.com/alecthomas/[email protected]/repr.go:402 +0x130
github.com/alecthomas/assert/v2.objectsAreEqual({0x57d180, 0x0}, {0x57d180, 0x0}, {0xc000076618?, 0xc000076620?, 0xc0000765f0?})
	/home/scott/dev/assert/assert.go:358 +0x152
github.com/alecthomas/assert/v2.Equal[...]({0x5e82d0, 0xc00000eb88}, {0xc000076700}, {0x4185e5?}, {0x0?, 0x59fdc0?, 0xc000076601?})
	/home/scott/dev/assert/assert.go:85 +0x365
github.com/alecthomas/assert/v2.TestEqual.func9({0x5e82d0?, 0xc00000eb88?})
	/home/scott/dev/assert/assert_test.go:49 +0x38
github.com/alecthomas/assert/v2.assertOk.func1(0xc0000eac40)
	/home/scott/dev/assert/assert_test.go:259 +0x6c
testing.tRunner(0xc0000eac40, 0xc000026c00)
	/usr/local/go/src/testing/testing.go:1934 +0xea
created by testing.(*T).Run in goroutine 6
	/usr/local/go/src/testing/testing.go:1997 +0x465
FAIL	github.com/alecthomas/assert/v2	0.005s
FAIL