Fix cursor go_up()/go_down(), turn on unit tests

src/core/Buffer.h

@@ -12,6 +12,7 @@ class Buffer
 public:
     Buffer(const char * filename = nullptr);
     bool write_to_file(const char * filename);
+    auto add_cursor() { return m_gap_buffer->add_cursor(); }
 
 protected:
     bool m_eol_at_eof;

src/core/GapBuffer.cc

@@ -223,6 +223,7 @@ bool GapBuffer::Cursor::go_up(size_t target_column)
     c2.go_start_of_line();
     *this = c2;
     m_line--;
+    m_column = 0u;
     forward_to_column(target_column);
     return true;
 }
@@ -237,6 +238,7 @@ bool GapBuffer::Cursor::go_down(size_t target_column)
     }
     *this = c2;
     m_line++;
+    m_column = 0u;
     forward_to_column(target_column);
     return true;
 }

src/core/GapBuffer.h

@@ -76,6 +76,8 @@ public:
         uint32_t operator*() const { return *m_iterator; }
         uint8_t * address() const { return m_iterator.address(); }
         bool valid() const { return m_iterator.valid(); }
+        size_t line() const { return m_line; }
+        size_t column() const { return m_column; }
 
     protected:
         Iterator m_iterator;