[lldb][Test] TestRerunAndExpr.py: explicitly delete a.out before rebuilding it

[Michael137]

Summary

Summary

Older versions of make would occasionally fail to realize
that a pre-requisite for the a.out target has changed. This
resulted in roughly 1 out of 10 test runs to fail. Instead of
relying on make to resolve this dependency simply remove the
file before rebuilding; this will give make no option but to
remake a.out.

Testing

• Confirmed that the test passes on the host for 100 runs where without the patch it would fail after ~10

Details

Adding -d to lldbtest's make invocation and running the
test without this patch sometimes yielded:

Removing child 0x600000308ff0 PID 19915 from chain.
    Successfully remade target file `rebuild.o'.
   Finished prerequisites of target file `a.out'.
   Prerequisite `rebuild.o' is newer than target `a.out'.
  No need to remake target `a.out'.

[Michael137]

• Reword comment

lldb/test/API/functionalities/rerun_and_expr/TestRerunAndExpr.py

def test(self):
         target.BreakpointCreateBySourceRegex('return', lldb.SBFileSpec('rebuild.cpp'))
         target.BreakpointCreateBySourceRegex('return', lldb.SBFileSpec('main.cpp'))
         process = target.LaunchSimple(None, None, self.get_process_working_directory())  
 
         self.expect_expr('foo', result_type='Foo', result_children=[
                 ValueCheck(name='m_val', value='42')
             ])
 
+        # Delete the executable to force make to rebuild it.
+        remove_file(exe)
         self.build(dictionary={'CXX_SOURCES':'rebuild.cpp', 'EXE':'a.out'})
 
         # Rerun program within the same target
         process.Destroy()
         process = target.LaunchSimple(None, None, self.get_process_working_directory())  
 
         self.expect_expr('foo', result_type='Foo', result_children=[
             ValueCheck(name='Base', children=[