test: Add testenv_enter.sh to ease debugging testenv

The script testenv_enter.sh, by default, runs /bin/sh in the environment
currently setup.

A bit easier than copy pasting the old hint text printed before test run.

Signed-off-by: Magnus Malm <magnusmalm@gmail.com>

test/testenv_start.sh

@@ -5,7 +5,7 @@ set -eu
 TEST_DIR=$(dirname "$0")
 TESTS_ROOT="$(pwd)/${TEST_DIR}/test_root"
 
->&2 echo "Hint: Execute '${TEST_DIR}/testenv_exec.sh \$(pgrep -P $$) /bin/sh; reset' to enter the test namespace"
+>&2 echo "Hint: Execute 'test/testenv_enter.sh' to enter the test namespace"
 
 unshare=$(command -v unshare)
 chroot=$(command -v chroot)