misc: Suggest usage of ALL instead of NULL in TESTING.md

We can now reallty build *all* unit tests using ALL.  The issue with
NULL is that some ISA specific unit tests were not actually run
therefore the user was not really running all available unittests

Change-Id: I4aab8e9a093d1f6ef4d473b0dbe240bf72dc739b
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/64311
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
diff --git a/TESTING.md b/TESTING.md
index f5d8198..2273e31 100644
--- a/TESTING.md
+++ b/TESTING.md
@@ -15,7 +15,7 @@
 To build and run all the unit tests:
 
 ```shell
-scons build/NULL/unittests.opt
+scons build/ALL/unittests.opt
 ```
 
 All unit tests should be run prior to posting a patch to
@@ -25,20 +25,20 @@
 `src/base/bitunion.test.cc`):
 
 ```shell
-scons build/NULL/base/bitunion.test.opt
-./build/NULL/base/bitunion.test.opt
+scons build/ALL/base/bitunion.test.opt
+./build/ALL/base/bitunion.test.opt
 ```
 
 To list the available test functions from a test file:
 
 ```shell
-./build/NULL/base/bitunion.test.opt --gtest_list_tests
+./build/ALL/base/bitunion.test.opt --gtest_list_tests
 ```
 
 To run a specific test function (e.g., BitUnionData.NormalBitfield):
 
 ```shell
-./build/NULL/base/bitunion.test.opt --gtest_filter=BitUnionData.NormalBitfield
+./build/ALL/base/bitunion.test.opt --gtest_filter=BitUnionData.NormalBitfield
 ```
 
 # Running system-level tests