patch 8.2.3637: typos in test files

Problem:    Typos in test files.
Solution:   Correct the typos. (Dominique Pellé, closes #9175)
diff --git a/src/testdir/test_debugger.vim b/src/testdir/test_debugger.vim
index 957f5ac..86d0b4c 100644
--- a/src/testdir/test_debugger.vim
+++ b/src/testdir/test_debugger.vim
@@ -500,7 +500,7 @@
 
   call RunDbgCmd( buf, 'down', [ 'frame is zero' ] )
 
-  " step until we have another meaninfgul trace
+  " step until we have another meaningful trace
   call RunDbgCmd(buf, 'step', ['line 5: func File2Function()'])
   call RunDbgCmd(buf, 'step', ['line 9: call File2Function()'])
   call RunDbgCmd(buf, 'backtrace', [
@@ -588,7 +588,7 @@
                 \ ['cmd: doautocmd User TestGlobalFunction'])
   call RunDbgCmd(buf, 'step', ['cmd: call GlobalFunction() | echo "Done"'])
 
-  " At this point the ontly thing in the stack is the autocommand
+  " At this point the only thing in the stack is the autocommand
   call RunDbgCmd(buf, 'backtrace', [
         \ '>backtrace',
         \ '->0 User Autocommands for "TestGlobalFunction"',
@@ -718,7 +718,7 @@
 
   call RunDbgCmd( buf, 'down', [ 'frame is zero' ] )
 
-  " step until we have another meaninfgul trace
+  " step until we have another meaningful trace
   call RunDbgCmd(buf, 'step', ['line 5: func File2Function()'])
   call RunDbgCmd(buf, 'step', ['line 9: call File2Function()'])
   call RunDbgCmd(buf, 'backtrace', [
@@ -845,7 +845,7 @@
   call CheckDbgOutput(buf, ['command line',
                             \ 'cmd: call GlobalFunction()'], #{msec: 5000})
 
-  " At this point the ontly thing in the stack is the cmdline
+  " At this point the only thing in the stack is the cmdline
   call RunDbgCmd(buf, 'backtrace', [
         \ '>backtrace',
         \ '->0 command line',
@@ -1260,7 +1260,7 @@
         \ #{ match: 'pattern' } )
 
   " Expression evaluation in the script frame (not the function frame)
-  " FIXME: Unexpected in this scope (a: should not be visibnle)
+  " FIXME: Unexpected in this scope (a: should not be visible)
   call RunDbgCmd(buf, 'echo a:arg', [ 'arg1' ] )
   call RunDbgCmd(buf, 'echo s:file1_var', [ 'file1' ] )
   call RunDbgCmd(buf, 'echo g:global_var', [ 'global' ] )