test: add assertion for Close

Add an assertion for calling .Close() and ensuring no error is returned.

internal/assertions/assertions.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -1044,6 +1045,15 @@ func FilePathValid(path string) (s string) {
 	return
 }
 
+func Close(c io.Closer) (s string) {
+	err := c.Close()
+	if err != nil {
+		s = "calling Close failed\n"
+		s += bullet("error: %v\n", err)
+	}
+	return
+}
+
 func StrEqFold(exp, val string) (s string) {
 	if !strings.EqualFold(exp, val) {
 		s = "expected strings to be equal ignoring case\n"

test.go

@@ -1,6 +1,7 @@
 package test
 
 import (
+	"io"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -597,6 +598,12 @@ func FilePathValid(t T, path string, settings ...Setting) {
 	invoke(t, assertions.FilePathValid(path), settings...)
 }
 
+// Close asserts c.Close does not cause an error.
+func Close(t T, c io.Closer) {
+	t.Helper()
+	invoke(t, assertions.Close(c))
+}
+
 // StrEqFold asserts exp and val are equivalent, ignoring case.
 func StrEqFold(t T, exp, val string, settings ...Setting) {
 	t.Helper()

test_test.go

@@ -1399,6 +1399,22 @@ func TestFilePathValid(t *testing.T) {
 	FilePathValid(tc, "foo/../bar")
 }
 
+type closer struct {
+	err error
+}
+
+func (c *closer) Close() error {
+	return c.err
+}
+
+func TestClose(t *testing.T) {
+	tc := newCase(t, `calling Close failed`)
+	t.Cleanup(tc.assert)
+
+	c := &closer{err: errors.New("oops")}
+	Close(tc, c)
+}
+
 func TestStrEqFold(t *testing.T) {
 	tc := newCase(t, `expected strings to be equal ignoring case`)
 	t.Cleanup(tc.assert)