fix(mocktesting): add missing TempDirs() function

Without this function there was no way to inspect the list of temporary
directories created by calls to TempDir(), also making it impossible to
verify that TempDir() has been called.

t.go

@@ -424,3 +424,18 @@ func (t *T) Subtests() []*T {
 
 	return t.subtests
 }
+
+// TempDirs returns a string slice of temporary directories created by
+// TempDir().
+func (t *T) TempDirs() []string {
+	if t.tempdirs == nil {
+		t.mux.Lock()
+		t.tempdirs = []string{}
+		t.mux.Unlock()
+	}
+
+	t.mux.RLock()
+	defer t.mux.RUnlock()
+
+	return t.tempdirs
+}

t_test.go

@@ -2458,3 +2458,46 @@ func TestT_Subtests(t *testing.T) {
 		})
 	}
 }
+
+func TestT_TempDirs(t *testing.T) {
+	type fields struct {
+		tempdirs []string
+	}
+	tests := []struct {
+		name   string
+		fields fields
+		want   []string
+	}{
+		{
+			name:   "nil",
+			fields: fields{tempdirs: nil},
+			want:   []string{},
+		},
+		{
+			name:   "empty",
+			fields: fields{tempdirs: []string{}},
+			want:   []string{},
+		},
+		{
+			name:   "one dir",
+			fields: fields{tempdirs: []string{"/tmp/foo"}},
+			want:   []string{"/tmp/foo"},
+		},
+		{
+			name: "many dirs",
+			fields: fields{
+				tempdirs: []string{"/tmp/foo", "/tmp/foo", "/tmp/nope"},
+			},
+			want: []string{"/tmp/foo", "/tmp/foo", "/tmp/nope"},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			mt := &T{tempdirs: tt.fields.tempdirs}
+
+			got := mt.TempDirs()
+
+			assert.Equal(t, tt.want, got)
+		})
+	}
+}