Refactor test framework into a vim plugin.

tests/breakpoints.test.vim

@@ -1,5 +1,3 @@
-source lib/shared.vim
-
 function! SetUp()
   if exists ( 'g:loaded_vimpector' )
     unlet g:loaded_vimpector

tests/lib/plugin/screendump.vim

@@ -17,8 +17,6 @@ if !has('terminal')
   finish
 endif
 
-source lib/shared.vim
-
 " Run Vim with "arguments" in a new terminal window.
 " By default uses a size of 20 lines and 75 columns.
 " Returns the buffer number of the terminal.

tests/lib/run_test.vim

@@ -28,7 +28,7 @@
 " When debugging a test it can be useful to add messages to v:errors:
 "	call add(v:errors, "this happened")
 
-set rtp=$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after
+set rtp=$PWD/lib,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after
 if has('packages')
   let &packpath = &rtp
 endif

tests/tabpage.test.vim

@@ -1,5 +1,3 @@
-source lib/shared.vim
-
 function! SetUp()
   if exists ( 'g:loaded_vimpector' )
     unlet g:loaded_vimpector