tests: add a test_ prefix to all files that contain actual tests

This will make it easier to differentiate with utility files, as the next change
will add sevral

tests/Makefile

@@ -6,14 +6,14 @@ rawtests = \
 	test_here_doc \
 	test_wordlist_idx \
 	test_redirection_parsing \
-	quote_removal \
-	cmdlist_use_after_free \
-	metacharacters \
-	parse_command_lists \
-	parse_pipelines \
-	parse_simple_cmds \
+	test_quote_removal \
+	test_cmdlist_use_after_free \
+	test_metacharacters \
+	test_parse_command_lists \
+	test_parse_pipelines \
+	test_parse_simple_cmds \
 	test_env_manip \
-	word_splitting \
+	test_word_splitting \
 
 ifeq ($(CFLAGS),)
 	CFLAGS = -Wall -Wextra -Werror -g