(PUP-1897) Change epp lexer/grammar to only accept single rendered expr

It was confusing to allow more than a single expression in an epp
render expression - e.g. <%= 1 2 3 %> where only the first
expression was rendered, and the subsequent expressions only
evaluated for side effect.

This change restricts the <%= tag to only accept a single expression.
(PUP-1897) Fix failing lexer2 test, and update with additional tests

Tests were failing due to additional token marking the end of
an EPP render expression.
(PUP-1897) Improve test of EPP trim, and expr after params

This adds tests for render single expression with trim, and for
sequence of expressions after opening parameters.
Merge pull request #2424 from hlindberg/pup-1897_render-single-expression

(PUP-1897) Only accept single expression in EPP render expression