testutil/pkgdata: Add MakeTestDeb()

[woky]

Add MakeTestDeb() function to testutil/pkgdata. This function is just
like MakeDeb() except that it doesn't return an error. Instead, it
panics on errors.

This is useful in tests where package data is created in top-level test
case structures where we don't need to handle the error, e.g.:

var testCases = []TestCase{{
	pkg: MakeTestDeb([]TarEntry{
		...
	}),
}, {
	...
}}

[woky]

Changed the name of the function.

[jnsgruk]

Requested changes all made, thanks @woky - I'll merge this now.