网友的这个句子，语法上称为假拟分裂句，属于一种强调的手段。

通常所说的强调句型，it is...that...语法上称为分裂句。

He bought a new bike yesterday. 这是一个普通的陈述句。如果我们想强调宾语，我们既可以用分裂句，也可以用假拟分裂句来强调。

It was a new bike that he bought yesterday. 分裂句（即强调句型）

What he bought yesterday was a new bike. 假拟分裂句。

假拟分裂句的主语是一个名词性关系分句，表语是确认性表语，即主语=表语。所以主语和表语可以互换句法功能：即主语作表语，表语作主语。

A new bike was what he bought yesterday.

网友的句子就是主语和表语互换句法功能的假拟分裂句。所以句子的主语和表语可以互换位置：

A testimony to endurance is what she called it. what=the thing which，在名词性关系分句中作宾补。

A testimony to endurance is the thing which she called it.

What she called it is a testimony to endurance. 假拟分裂句强调宾语补足语。

The thing which she called it is a testimony to endurance.

如果不强调，还原成普通陈述句则为：

She called it a testimony to dundurance.

我们还可以用分裂句来强调宾补：

It was a testimony to endurance that she called it.