test(tc): test for AP/RSS NV counter interface

Change in PLATFORM_TEST build flag from boolean -> string, with the
current string options being tfm-testsuite and rss-nv-counters.

To get the old behavior, i.e. where we used to use PLATFORM_TEST=1,
we now need to pass PLATFORM_TEST=tfm-testsuite.

Adding new test of the AP/RSS interface for non-volatile counters.
The test reads, increments, and reads again each 3 types of NV
counters for: CCA, secure, and non-secure firmware. Enabled by
PLATFORM_TEST=rss-nv-counters.

Change-Id: I2044cc9b2f37984697e0754c9c824eab51a11e7f
Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/plat/arm/board/tc/platform_test.mk b/plat/arm/board/tc/platform_test.mk
index c2ee69e..5cdbb86 100644
--- a/plat/arm/board/tc/platform_test.mk
+++ b/plat/arm/board/tc/platform_test.mk
@@ -1,9 +1,9 @@
-# Copyright (c) 2022, Arm Limited. All rights reserved.
+# Copyright (c) 2022-2023, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
 
-ifeq (${PLATFORM_TEST},1)
+ifeq (${PLATFORM_TEST},tfm-testsuite)
 
     # The variables need to be set to compile the platform test:
     ifeq (${TF_M_TESTS_PATH},)