subsys/testsuite: rearrange for standard use of extern "C"

Consistently place C++ use of extern "C" after all include directives,
within the negative branch of _ASMLANGUAGE if used.

Background from issue #17997:

Declarations that use C linkage should be placed within extern "C"
so the language linkage is correct when the header is included by
a C++ compiler.

Similarly #include directives should be outside the extern "C" to
ensure the language-specific default linkage is applied to any
declarations provided by the included header.

See: https://en.cppreference.com/w/cpp/language/language_linkage
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

subsys/testsuite/ztest/include/ztest.h

@@ -20,10 +20,6 @@
 #ifndef __ZTEST_H__
 #define __ZTEST_H__
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 /**
  * @defgroup ztest Zephyr testing suite
  */
@@ -44,21 +40,30 @@ extern "C" {
 /* FIXME: Properly integrate with Zephyr's arch specific code */
 #define CONFIG_X86 1
 #define CONFIG_PRINTK 1
+#ifdef __cplusplus
+extern "C" {
+#endif
 struct esf;
 typedef struct esf z_arch_esf_t;
+#ifdef __cplusplus
+}
 #endif
+#endif /* KERNEL */
 
 #include <sys/printk.h>
 #define PRINT printk
 
 #include <zephyr.h>
 
-
 #include <ztest_assert.h>
 #include <ztest_mock.h>
 #include <ztest_test.h>
 #include <tc_util.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void test_main(void);
 
 #ifdef __cplusplus

subsys/testsuite/ztest/include/ztest_assert.h

@@ -19,6 +19,10 @@
 #include <stdio.h>
 #include <string.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void ztest_test_fail(void);
 #if CONFIG_ZTEST_ASSERT_VERBOSE == 0
 
@@ -214,4 +218,8 @@ static inline void z_zassert(int cond,
  * @}
  */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* __ZTEST_ASSERT_H__ */

subsys/testsuite/ztest/include/ztest_mock.h

@@ -91,6 +91,10 @@
 
 #include <zephyr/types.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void z_init_mock(void);
 int z_cleanup_mock(void);
 
@@ -101,6 +105,10 @@ void z_ztest_check_expected_value(const char *fn, const char *param,
 void z_ztest_returns_value(const char *fn, uintptr_t value);
 uintptr_t z_ztest_get_return_value(const char *fn);
 
+#ifdef __cplusplus
+}
+#endif
+
 #else /* !CONFIG_ZTEST_MOCKING */
 
 #define z_init_mock()

subsys/testsuite/ztest/include/ztest_test.h

@@ -15,6 +15,10 @@
 
 #include <app_memory/app_memdomain.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct unit_test {
 	const char *name;
 	void (*test)(void);
@@ -171,4 +175,8 @@ extern struct k_mem_domain ztest_mem_domain;
  * @}
  */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* __ZTEST_ASSERT_H__ */