tests/posix/fs: Add test for fdopen

Add a test to the POSIX filesystem API suite that checks whether fdopen()
returns a valid value.

Signed-off-by: Jakub Klimczak <jklimczak@internships.antmicro.com>
Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>

Author: Jakub Klimczak

tests/posix/fs/src/test_fs_file.c

@@ -28,6 +28,18 @@ static int test_file_open(void)
 	return TC_PASS;
 }
 
+static int test_file_fdopen(void)
+{
+	FILE *fp = fdopen(file, "r");
+
+	if (fp == NULL) {
+		TC_ERROR("Failed associating file descriptor %d with FILE, errno=%d\n", file,
+			 errno);
+		return TC_FAIL;
+	}
+	return TC_PASS;
+}
+
 int test_file_write(void)
 {
 	ssize_t brw;
@@ -197,6 +209,18 @@ ZTEST(posix_fs_file_test, test_fs_open)
 	zassert_true(test_file_open() == TC_PASS);
 }
 
+/**
+ * @brief Test for POSIX fdopen API
+ *
+ * @details Test converts file descriptor to FILE * through POSIX fdopen API.
+ */
+ZTEST(posix_fs_file_test, test_fs_fdopen)
+{
+	/* FIXME: restructure tests as per #46897 */
+	zassert_true(test_file_open() == TC_PASS);
+	zassert_true(test_file_fdopen() == TC_PASS);
+}
+
 /**
  * @brief Test for POSIX write API
  *