Client-side and server-side validation of test file upload

[quasimik]

Server validation errors should return appropriate error codes.

Referring to this, probably the only relevant ones are:

• 400 Bad Request when fields are missing or malformed
• 413 Payload Too Large when fields are too large (including files)

For 413 Payload Too Large, maybe multer already somehow takes care of it (needs testing):

UPE-Website-V2/backend/server.js

Lines 72 to 80 in 124c092

	limits: {
	// fieldNameSize: asdf,
	fieldSize: TEST_FILE_UPLOAD_MAX_SIZE,
	// fields: asdf,
	fileSize: TEST_FILE_UPLOAD_MAX_SIZE,
	// files: asdf,
	// parts: asdf,
	// headerPairs: asdf,
	}

Remember that authorization is already handled by the verifyToken middleware, so we shouldn't have to bother with a 403 Forbidden here.