tst_test.sh: Replace GNU word boundary \b with \< in grep

The regex word boundary extension '\b' is a GNU extension and is not
supported by Toybox and lightweight POSIX regex implementations
(such as Bionic libc), causing grep to fail with:

    grep: bad regex '^[^#]*\bTST_': trailing backslash (\)

Replace '\b' with '\<' to ensure shell tests run correctly on
lightweight environments such as Android (Toybox).

Link: https://lore.kernel.org/ltp/20260630071639.2248082-1-wakel@google.com/
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Terence Tritton <ttritton@google.com>
Co-developed-by: Wake Liu <wakel@google.com>
Signed-off-by: Wake Liu <wakel@google.com>

Author: Terence Tritton

testcases/lib/tst_test.sh

@@ -676,7 +676,7 @@ tst_run()
 	local ret
 
 	if [ -n "$TST_TEST_PATH" ]; then
-		for _tst_i in $(grep '^[^#]*\bTST_' "$TST_TEST_PATH" | sed "s/.*TST_//; s/$_tst_pattern//"); do
+		for _tst_i in $(grep '^[^#]*\<TST_' "$TST_TEST_PATH" | sed "s/.*TST_//; s/$_tst_pattern//"); do
 			case "$_tst_i" in
 			ALL_FILESYSTEMS|DISABLE_APPARMOR|DISABLE_SELINUX);;
 			SETUP|CLEANUP|TESTFUNC|ID|CNT|MIN_KVER);;
@@ -698,7 +698,7 @@ tst_run()
 			esac
 		done
 
-		for _tst_i in $(grep '^[^#]*\b_tst_' "$TST_TEST_PATH" | sed "s/.*_tst_//; s/$_tst_pattern//"); do
+		for _tst_i in $(grep '^[^#]*\<_tst_' "$TST_TEST_PATH" | sed "s/.*_tst_//; s/$_tst_pattern//"); do
 			tst_res TWARN "Private variable or function _tst_$_tst_i used!"
 		done
 	fi