test: fix typos in read-buffer tests

The offset-exceeding tests for readFloat contained a double test
for readFloatLE instead of one for readFloatLE and one for
readFloatBE. This is fixed in this commit.

PR-URL: https://github.com/nodejs/node/pull/16834


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>