tests: avoid a FP on aarch64 when comparing env/printenv output

tests: avoid a FP on aarch64 when comparing env/printenv output

The following test fails on aarch64 on openSUSE's OpenBuildService
due to glibc's execvp reversing the pointers of 'environ', i.e.,
the output of "env|tac" equals "env env" on that platform.

* tests/misc/printenv.sh: Use 'env env' to work around the behavior
on that platform.
While at it, fix the grep pattern which suppressed all environment
variables starting with an underscore "_" instead of "$_" (and
"$LD_PRELOAD") only.