Skip creating args for case when only one TestParams

[mekegi]

A lot of cases when func with one param func Sin(x int) int,
or writers func Write(w io.Writer, b []bytes)
and in generated tests for such func we see a lot of boilercode like
type args struct { x int }
In my pull request I try ti prevent creating unuseful args type for one param
Now generated tests looks like:

	tests := []struct {
		name string
		arg  string
	}{
	// TODO: Add test cases.
	}
	for _, tt := range tests {
		Foo3(tt.arg)
	}

instead of

	type args struct {
		s string
	}
	tests := []struct {
		name string
		args args
	}{
	// TODO: Add test cases.
	}
	for _, tt := range tests {
		Foo3(tt.args.s)
	}

[coveralls]

Coverage remained the same at 96.327% when pulling 45ff4bc on mekegi:master into a680bea on cweill:master.