bufio: test for exact error value in TestNegativeEOFReader and TestLargeReader

CL 225357 added tests for Scanner not panicking on bad readers.
CL 225557 created a named error value that is returned instead.
CL 237739 documents that the bufio.ErrBadReadCount is returned
when bufio.Scanner is used with an invalid io.Reader.

This suggests we wouldn't want that behavior to be able to change
without a test noticing it, so modify the tests to check for the
exact error value instead of just any non-nil one.

For #38053.

Change-Id: I4b0b8eb6804ebfe2c768505ddb94f0b1017fcf8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/238217


Reviewed-by: Ian Lance Taylor <iant@golang.org>