patch 8.1.1581: shared functions for testing are disorganised

Problem:    Shared functions for testing are disorganised.
Solution:   Group finctions in script files. (Ozaki Kiichi, closes #4573)
diff --git a/src/testdir/test_mksession.vim b/src/testdir/test_mksession.vim
index e7aa25c..078b8a0 100644
--- a/src/testdir/test_mksession.vim
+++ b/src/testdir/test_mksession.vim
@@ -7,6 +7,7 @@
 CheckFeature mksession
 
 source shared.vim
+source term_util.vim
 
 func Test_mksession()
   tabnew
@@ -338,7 +339,7 @@
   endfor
   call assert_match('terminal ++curwin ++cols=\d\+ ++rows=\d\+\s*.*$', term_cmd)
 
-  call Stop_shell_in_terminal(bufnr('%'))
+  call StopShellInTerminal(bufnr('%'))
   call delete('Xtest_mks.out')
 endfunc
 
@@ -353,7 +354,7 @@
     endif
   endfor
 
-  call Stop_shell_in_terminal(bufnr('%'))
+  call StopShellInTerminal(bufnr('%'))
   call delete('Xtest_mks.out')
 endfunc
 
@@ -368,7 +369,7 @@
     endif
   endfor
 
-  call Stop_shell_in_terminal(bufnr('%'))
+  call StopShellInTerminal(bufnr('%'))
   call delete('Xtest_mks.out')
 endfunc
 
@@ -384,7 +385,7 @@
     endif
   endfor
 
-  call Stop_shell_in_terminal(bufnr('%'))
+  call StopShellInTerminal(bufnr('%'))
   call delete('Xtest_mks.out')
 endfunc
 
@@ -400,7 +401,7 @@
     endif
   endfor
 
-  call Stop_shell_in_terminal(bufnr('%'))
+  call StopShellInTerminal(bufnr('%'))
   call delete('Xtest_mks.out')
   set sessionoptions&
 endfunc
@@ -418,7 +419,7 @@
   endfor
   call assert_match('terminal ++curwin ++cols=\d\+ ++rows=\d\+.*other', term_cmd)
 
-  call Stop_shell_in_terminal(bufnr('%'))
+  call StopShellInTerminal(bufnr('%'))
   call delete('Xtest_mks.out')
 endfunc