[flang][test] Share all driver test dirs between f18 and flang-new

Originally, when we added the new driver, we created dedicated test
directories for `flang-new`. This way we separated the tests for the
`throwaway` and the new driver.

As we are increasing test coverage and starting to share tests between
the two drivers, it makes sense to share all directories and instead
rely on:
```
! REQUIRES: new-flang-driver
```
to mark tests as exclusively for the new driver.

Differential Revision: https://reviews.llvm.org/D97207

flang/test/Frontend/prescanner-diag.f90

@@ -1,17 +1,15 @@
 ! Test that the driver correctly reports diagnostics from the prescanner. The contents of the include file are irrelevant here.
 
 ! Test with -E (i.e. PrintPreprocessedAction, stops after prescanning)
-! RUN: %f18 -E -I %S/Inputs/ %s 2>&1 | FileCheck %s
-! RUN: %flang-new -E -I %S/Inputs/ %s 2>&1 | FileCheck %s
-! RUN: %flang-new -fc1 -E -I %S/Inputs/ %s 2>&1 | FileCheck %s
+! RUN: %flang -E -I %S/Inputs/ %s 2>&1 | FileCheck %s
+! RUN: %flang_fc1 -E -I %S/Inputs/ %s 2>&1 | FileCheck %s
 
 ! Test with -fsyntax-only (i.e. ParseSyntaxOnlyAction, stops after semantic checks)
-! RUN: %f18 -fsyntax-only -I %S/Inputs/ %s 2>&1 | FileCheck %s
-! RUN: %flang-new -fsyntax-only -I %S/Inputs/ %s 2>&1 | FileCheck %s
-! RUN: %flang-new -fc1 -fsyntax-only -I %S/Inputs/ %s 2>&1 | FileCheck %s
+! RUN: %flang -fsyntax-only -I %S/Inputs/ %s 2>&1 | FileCheck %s
+! RUN: %flang_fc1 -fsyntax-only -I %S/Inputs/ %s 2>&1 | FileCheck %s
 
-! CHECK: prescanner-diag.f90:14:20: #include: extra stuff ignored after file name
+! CHECK: prescanner-diag.f90:12:20: #include: extra stuff ignored after file name
 #include <empty.h> comment
-! CHECK: prescanner-diag.f90:16:20: #include: extra stuff ignored after file name
+! CHECK: prescanner-diag.f90:14:20: #include: extra stuff ignored after file name
 #include "empty.h" comment
 end

flang/test/lit.cfg.py

@@ -39,12 +39,9 @@
 config.excludes = ['Inputs', 'CMakeLists.txt', 'README.txt', 'LICENSE.txt']
 
 # If the new Flang driver is enabled, add the corresponding feature to
-# config. Otherwise, exclude the corresponding test directory.
+# config.
 if config.include_flang_new_driver_test:
   config.available_features.add('new-flang-driver')
-else:
-  config.excludes.append('Flang-Driver')
-  config.excludes.append('Frontend')
 
 # test_source_root: The root path where tests are located.
 config.test_source_root = os.path.dirname(__file__)