Filter out _AST_FEATURES when comparing environment state. (#110283)

Filter out _AST_FEATURES when comparing environment state. (#110283)

ksh93 uses an environment variable called _AST_FEATURES to communicate
with subshell instances of itself.  Its value may change at any time
so AT_CHECK_ENV and AT_CONFIG_CMP should ignore it.

This was responsible for many spurious testsuite failures on
OpenIndiana.  Problem reported by Bob Friesenhahn.

* tests/local.at (_AT_CHECK_ENV, AT_CONFIG_CMP): Add _AST_FEATURES to
  list of variables set by shells to unstable values.