LibCore: Fix corner case for files without newlines

When BufferedFile.can_read_line() was invoked on files with no newlines,
t incorrectly returned a false result for this single line that, even
though doesn't finish with a newline character, is still a line. Since
this method is usually used in tandem with read_line(), users would miss
reading this line (and hence all the file contents).

This commit fixes this corner case by adding another check after a
negative result from finding a newline character. This new check does
the same as the check that is done *before* looking for newlines, which
takes care of this problem, but only works for files that have at least
one newline (hence the buffer has already been filled).

A new unit test has been added that shows the use case. Without the
changes in this commit the test fails, which is a testament that this
commit really fixes the underlying issue.

AK/BufferedStream.h

@@ -193,7 +193,10 @@ public:
         if (stream().is_eof())
             return m_buffer.used_space() > 0;
 
-        return TRY(find_and_populate_until_any_of(Array<StringView, 1> { "\n"sv })).has_value();
+        auto maybe_match = TRY(find_and_populate_until_any_of(Array { "\n"sv }));
+        if (maybe_match.has_value())
+            return true;
+        return stream().is_eof() && m_buffer.used_space() > 0;
     }
 
     bool is_eof() const

Tests/LibCore/TestLibCoreStream.cpp

@@ -553,6 +553,24 @@ TEST_CASE(buffered_file_tell_and_seek)
     }
 }
 
+constexpr auto new_newlines_message = "Hi, look, no newlines"sv;
+
+TEST_CASE(buffered_file_without_newlines)
+{
+    constexpr auto filename = "/tmp/file-without-newlines"sv;
+    auto file_wo_newlines = Core::File::open(filename, Core::File::OpenMode::Write).release_value();
+    EXPECT(!file_wo_newlines->write_until_depleted(new_newlines_message.bytes()).is_error());
+    file_wo_newlines->close();
+
+    auto ro_file = Core::BufferedFile::create(Core::File::open(filename, Core::File::OpenMode::Read).release_value(), new_newlines_message.length() + 1).release_value();
+
+    auto maybe_can_read_line = ro_file->can_read_line();
+    EXPECT(!maybe_can_read_line.is_error());
+    EXPECT(maybe_can_read_line.release_value());
+    Array<u8, new_newlines_message.length() + 1> buffer;
+    EXPECT(ro_file->read_line(buffer).release_value() == new_newlines_message);
+}
+
 constexpr auto buffered_sent_data = "Well hello friends!\n:^)\nThis shouldn't be present. :^("sv;
 constexpr auto first_line = "Well hello friends!"sv;
 constexpr auto second_line = ":^)"sv;