Running truffle test from the command line should import the Assert.sol library if truffle was installed globally (using npm install -g truffle).

If truffle is globally installed, the "Assert.sol" file should be located in the directory

C:\Users\[YourUsername]\AppData\Roaming\npm\node_modules\truffle\build

If things still don't work you could try copying the "Assert.sol" (and its imported dependencies, of which there's several) to your "contracts" folder and then importing it from there into your test contract. But it's probably best to just get truffle installed globally and then use truffle test.