Simple test for checking if `omitBasePath` implementation worked as expected. My PR submission for apex/gateway.

request_test.go

package gateway

import "testing"

// omitBasePath strips out the base path from the given path.
//
// It allows to support both API endpoints (default, auto-generated
// "execute-api" address and configured Base Path Mapping
// with a Custom Domain Name), while preserving the same routing
// registered on the http.Handler.
func omitBasePath(path string, basePath string) string {
	if path == "/" || basePath == "" {
		return path
	}

	if strings.HasPrefix(path, "/"+basePath) {
		path = strings.Replace(path, basePath, "", 1)
	}
	if strings.HasPrefix(path, "//") {
		path = path[1:]
	}

	return path
}

func Test_omitBasePath(t *testing.T) {
    basePath := "items"

    type args struct {
        path     string
        basePath string
    }
    tests := []struct {
        name string
        args args
        want string
    }{
        {"ListItems_WithBasePath_Default", args{"/", basePath}, "/"},
        {"ListItems_WithBasePath_Custom", args{"/items", basePath}, "/"},

        {"GetItem_WithBasePath_Default", args{"/123", basePath}, "/123"},
        {"GetItem_WithBasePath_Custom", args{"/items/123", basePath}, "/123"},

        {"ListItems_WithoutBasePath", args{"/", ""}, "/"},
        {"GetItem_WithoutBasePath", args{"/123", ""}, "/123"},
    }
    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            if got := omitBasePath(tt.args.path, tt.args.basePath); got != tt.want {
                t.Errorf("omitBasePath() = %v, want %v", got, tt.want)
            }
        })
    }
}

go test -v -timeout 30s github.com\piotrkubisa\gateway -run ^Test_omitBasePath$

=== RUN   Test_omitBasePath
=== RUN   Test_omitBasePath/ListItems_WithBasePath_Default
=== RUN   Test_omitBasePath/ListItems_WithBasePath_Custom
=== RUN   Test_omitBasePath/GetItem_WithBasePath_Default
=== RUN   Test_omitBasePath/GetItem_WithBasePath_Custom
=== RUN   Test_omitBasePath/ListItems_WithoutBasePath
=== RUN   Test_omitBasePath/GetItem_WithoutBasePath
--- PASS: Test_omitBasePath (0.00s)
    --- PASS: Test_omitBasePath/ListItems_WithBasePath_Default (0.00s)
    --- PASS: Test_omitBasePath/ListItems_WithBasePath_Custom (0.00s)
    --- PASS: Test_omitBasePath/GetItem_WithBasePath_Default (0.00s)
    --- PASS: Test_omitBasePath/GetItem_WithBasePath_Custom (0.00s)
    --- PASS: Test_omitBasePath/ListItems_WithoutBasePath (0.00s)
    --- PASS: Test_omitBasePath/GetItem_WithoutBasePath (0.00s)
PASS
ok   github.com/piotrkubisa/gateway  0.050s
Success: Tests passed.