text/template: 0xef is an integer, not a floating-point value.

text/template: 0xef is an integer, not a floating-point value.
The discriminator in the execution engine was stupid.
Add a test to the parse package too. The problem wasn't there
but the particular case ('e' in a hex integer) was not covered.

Fixes #8622.

LGTM=ruiu
R=golang-codereviews, ruiu
CC=golang-codereviews
https://golang.org/cl/133530043