patch 8.2.3914: various spelling mistakes in comments

Problem:    Various spelling mistakes in comments.
Solution:   Fix the mistakes. (Dominique Pellé, closes #9416)
diff --git a/src/testdir/test_debugger.vim b/src/testdir/test_debugger.vim
index 6f01917..e305033 100644
--- a/src/testdir/test_debugger.vim
+++ b/src/testdir/test_debugger.vim
@@ -948,7 +948,7 @@
 
   call RunDbgCmd(buf, 'step', ['line 1: SourceAnotherFile()'])
   call RunDbgCmd(buf, 'step', ['line 1: source Xtest2.vim'])
-  " Repeated line, because we fist are in the compiled function before the
+  " Repeated line, because we first are in the compiled function before the
   " EXEC and then in do_cmdline() before the :source command.
   call RunDbgCmd(buf, 'step', ['line 1: source Xtest2.vim'])
   call RunDbgCmd(buf, 'step', ['line 1: vim9script'])
diff --git a/src/testdir/test_source.vim b/src/testdir/test_source.vim
index 559208a..8f8e963 100644
--- a/src/testdir/test_source.vim
+++ b/src/testdir/test_source.vim
@@ -69,7 +69,7 @@
   call delete('Xfile.vim')
 endfunc
 
-" Test for expanding <sfile> in a autocmd and for <slnum> and <sflnum>
+" Test for expanding <sfile> in an autocmd and for <slnum> and <sflnum>
 func Test_source_autocmd_sfile()
   let code =<< trim [CODE]
     let g:SfileName = ''