A testament is strong proof or evidence of something, or a formal statement of what someone believes. It can also mean a person's will that explains what should happen to their property after they die, as in 'last will and testament'.
From Latin *testamentum* 'a will, a testament', from *testari* 'to be a witness, to testify'. The idea is that the document 'bears witness' to a person's wishes or beliefs.
Calling something 'a testament to your hard work' is like saying it’s a witness standing up in court for you. The 'Old Testament' and 'New Testament' are literally 'old' and 'new' witness-documents about God and people.
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