Test: Remove app header from test folder

Test should not include app header files. 'execute_ns_interactive_tests'
is only used in 'core_ns_interactive_testsuite.c', so remove the
declaration in 'app/tfm_integ_test.h' and change the function attribute
to 'static'. Thus, test doesn't need to include app header files.

Change-Id: I9cb2c3dd3eba2d718b998be85619235f2515481c
Signed-off-by: Summer Qin <summer.qin@arm.com>
diff --git a/app/tfm_integ_test.h b/app/tfm_integ_test.h
index 778f2ae..93c46a5 100644
--- a/app/tfm_integ_test.h
+++ b/app/tfm_integ_test.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2019, Arm Limited. All rights reserved.
+ * Copyright (c) 2017-2020, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -35,12 +35,6 @@
 void test_app(void *argument);
 #endif /* TEST_FRAMEWORK_NS */
 
-/**
- * \brief Execute the interactive test cases (button push)
- *
- */
-void execute_ns_interactive_tests(void);
-
 #ifdef __cplusplus
 }
 #endif