merge: improve string tests

 * test: improve test_mutt_str_adjust()
 * test: improve test_mutt_str_append_item()
 * test: improve test_mutt_str_asprintf()
 * test: improve test_mutt_str_dequote_comment()
 * test: improve test_mutt_str_find_word()
 * test: improve test_mutt_str_getenv()
 * test: improve test_mutt_str_inline_replace()
 * test: improve test_mutt_str_is_ascii()
 * test: improve test_mutt_str_is_email_wsp()
 * test: improve test_mutt_str_lws_len()
 * test: improve test_mutt_str_lws_rlen()
 * test: improve test_mutt_str_next_word()
 * test: improve test_mutt_str_pretty_size()
 * test: improve test_mutt_str_remall_strcasestr()
 * test: improve test_mutt_str_remove_trailing_ws()
 * test: improve test_mutt_str_replace()
 * test: improve test_mutt_str_rstrnstr()
 * test: improve test_mutt_str_skip_email_wsp()
 * test: improve test_mutt_str_skip_whitespace()
 * test: improve test_mutt_str_split()
 * test: improve test_mutt_str_startswith()
 * test: improve test_mutt_str_strcasecmp()
 * test: improve test_mutt_str_strcasestr()
 * test: improve test_mutt_str_strcat()
 * test: improve test_mutt_str_strchrnul()
 * test: improve test_mutt_str_strcmp()
 * test: improve test_mutt_str_strcoll()
 * test: improve test_mutt_str_strdup()
 * test: improve test_mutt_str_strfcpy()
 * test: improve test_mutt_str_stristr()
 * test: improve test_mutt_str_strlen()
 * test: improve test_mutt_str_strlower()
 * test: improve test_mutt_str_strncasecmp()
 * test: improve test_mutt_str_strncat()
 * test: improve test_mutt_str_strncmp()
 * test: improve test_mutt_str_strnfcpy()
 * test: improve test_mutt_str_substr_cpy()
 * test: improve test_mutt_str_substr_dup()
 * test: improve test_mutt_str_word_casecmp()