assert: make tHelper a type alias

The tHelper interface is defined only for internal usage. We don't need
a "hard" type for it as we don't define methods on it. Let's make is
just a alias to the anonymous interface it represents.

Note: we are already using type aliases elswhere, and type aliases were
introduced a long time ago in Go 1.9.

assert/assertions.go

@@ -1872,7 +1872,7 @@ var spewConfigStringerEnabled = spew.ConfigState{
 	MaxDepth:                10,
 }
 
-type tHelper interface {
+type tHelper = interface {
 	Helper()
 }
 

require/requirements.go

@@ -6,7 +6,7 @@ type TestingT interface {
 	FailNow()
 }
 
-type tHelper interface {
+type tHelper = interface {
 	Helper()
 }