ascii85_test.go contains a variable called bigtest that is used as
test data for TestDecoderBuffering and TestEncoderBuffering.  The
variable is initialised to a copy of the last element of the pairs
slice.  When the variable was first added the last element of this
slice contained a sizable test case, 342 encoded characters.  However,
https://golang.org/cl/5970078 added a new element to the end of the pairs
slice without updating bigtest.  As the new element contained only 1 byte
of encoded data bigtest became very small test.  This commit fixes the
problem by resetting bigtest to its original value and making its
initialisation independent of the layout of pairs.  All the unit tests
still pass.

Change-Id: If7fb609ced9da93a2321dfd8372986b2fa772fd5
Reviewed-on: https://go-review.googlesource.com/46475Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>