AK+Tests: Correct off-by-one error when right-trimming text

If the entire string you want to right-trim consists of characters you
want to remove, we previously would incorrectly leave the first
character there.

For example: `trim("aaaaa", "a")` would return "a" instead of "".

We can't use `i >= 0` in the loop since that would fail to detect
underflow, so instead we keep `i` in the range `size .. 1` and then
subtract 1 from it when reading the character.

Added some trim() tests while I was at it. (And to confirm that this was
the issue.)

AK/StringUtils.cpp

@@ -338,10 +338,10 @@ StringView trim(StringView str, StringView characters, TrimMode mode)
  }
 
  if (mode == TrimMode::Right || mode == TrimMode::Both) {
- for (size_t i = str.length() - 1; i > 0; --i) {
+ for (size_t i = str.length(); i > 0; --i) {
  if (substring_length == 0)
  return ""sv;
- if (!characters.contains(str[i]))
+ if (!characters.contains(str[i - 1]))
  break;
  --substring_length;
  }

Tests/AK/TestStringUtils.cpp

@@ -338,6 +338,17 @@ TEST_CASE(is_whitespace)
  EXPECT(!AK::StringUtils::is_whitespace("a\t"sv));
 }
 
+TEST_CASE(trim)
+{
+ EXPECT_EQ(AK::StringUtils::trim("aaa.a."sv, "."sv, TrimMode::Right), "aaa.a"sv);
+ EXPECT_EQ(AK::StringUtils::trim("...aaa"sv, "."sv, TrimMode::Left), "aaa"sv);
+ EXPECT_EQ(AK::StringUtils::trim("...aaa.a..."sv, "."sv, TrimMode::Both), "aaa.a"sv);
+ EXPECT_EQ(AK::StringUtils::trim("."sv, "."sv, TrimMode::Right), ""sv);
+ EXPECT_EQ(AK::StringUtils::trim("."sv, "."sv, TrimMode::Left), ""sv);
+ EXPECT_EQ(AK::StringUtils::trim("."sv, "."sv, TrimMode::Both), ""sv);
+ EXPECT_EQ(AK::StringUtils::trim("..."sv, "."sv, TrimMode::Both), ""sv);
+}
+
 TEST_CASE(find)
 {
  String test_string = "1234567";