✅ Add buffer appending unit tests

tests/units/autoload_proompter_lib.vader

@@ -0,0 +1,21 @@
+
+""
+" proompter#lib#ConcatenateWithLastLineOfBuffer {{{
+Given(One line already present):
+  Vim is
+Execute(proompter#lib#ConcatenateWithLastLineOfBuffer -- Complete the sentence):
+  call proompter#lib#ConcatenateWithLastLineOfBuffer(bufnr('[Vader-workbench]'), ' the best!')
+Expect(One line to be finished):
+  Vim is the best!
+
+Given(Two lines already present):
+  Vim is the best!
+  And
+Execute(proompter#lib#ConcatenateWithLastLineOfBuffer -- Inserts a new line after appending to existing content):
+  call proompter#lib#ConcatenateWithLastLineOfBuffer(bufnr('[Vader-workbench]'), " there is no\ndoubt about it")
+Expect(One line to be finished):
+  Vim is the best!
+  And there is no
+  doubt about it
+" }}}
+""