Fix an AttributeError in dotest.py if --executable points to a wrong place

This patch fixes the following case:
```
  $ ./dotest.py --executable=~/p/llvm/build_ninja/bin/lldb tools/lldb-mi/
  '~/p/llvm/build_ninja/bin/lldb' is not a path to a valid executable
  Traceback (most recent call last):
    File "./dotest.py", line 1306, in <module>
      setupSysPath()
    File "./dotest.py", line 1004, in setupSysPath
      if not lldbtest_config.lldbExec:
  AttributeError: 'module' object has no attribute 'lldbExec'
```

And with this fix:
```
  $ ./dotest.py --executable=~/p/llvm/build_ninja/bin/lldb tools/lldb-mi/
  '~/p/llvm/build_ninja/bin/lldb' is not a path to a valid executable
  The 'lldb' executable cannot be located.  Some of the tests may not be run as a result.
```

llvm-svn: 247256

1 files changed, 1 insertions, 1 deletions

diff --git a/lldb/test/dotest.py b/lldb/test/dotest.py
index fed8c0ba6ba..45d5c383979 100755
--- a/lldb/test/dotest.py
+++ b/lldb/test/dotest.py
@@ -997,7 +997,7 @@ def setupSysPath():
 
     if lldbtest_config.lldbExec and not is_exe(lldbtest_config.lldbExec):
         print "'{}' is not a path to a valid executable".format(lldbtest_config.lldbExec)
-        del lldbtest_config.lldbExec
+        lldbtest_config.lldbExec = None
 
     if not lldbtest_config.lldbExec:
         print "The 'lldb' executable cannot be located.  Some of the tests may not be run as a result."