tests: hoist envvar-check so it is run for every test

I.e., also for Perl-based tests; not just the ones using test-lib.sh.
* tests/check.mk (TESTS_ENVIRONMENT): Source envvar-check here, ...
* tests/test-lib.sh: ...not here.

tests/check.mk

@@ -43,8 +43,11 @@ check: vc_exe_in_TESTS
 built_programs = \
   (cd $(top_builddir)/src && MAKEFLAGS= $(MAKE) -s built_programs.list)
 
-# Append this, because automake does the same.
+# Note that the first line is a statement.  It ensures that environment
+# variables that can perturb tests are unset.  The rest are envvar settings
+# that propagate build-related Makefile variables to test scripts.
 TESTS_ENVIRONMENT =				\
+  . $(top_srcdir)/tests/envvar-check;		\
   LOCALE_FR='$(LOCALE_FR)'			\
   abs_top_builddir='$(abs_top_builddir)'	\
   abs_top_srcdir='$(abs_top_srcdir)'		\

tests/du/basic

@@ -21,7 +21,6 @@ if test "$VERBOSE" = yes; then
   du --version
 fi
 
-# DU_BLOCK_SIZE could cause problems
 . $top_srcdir/tests/lang-default
 . $top_srcdir/tests/test-lib.sh
 

tests/test-lib.sh

@@ -208,8 +208,6 @@ test_dir_=$(pwd)
 this_test_() { echo "./$0" | sed 's,.*/,,'; }
 this_test=$(this_test_)
 
-. $top_srcdir/tests/envvar-check
-
 # This is a stub function that is run upon trap (upon regular exit and
 # interrupt).  Override it with a per-test function, e.g., to unmount
 # a partition, or to undo any other global state changes.