tools/testing/nvdimm: force nfit_test to depend on instrumented modules

The libnvdimm unit tests will fail when they are run against the
production / in-tree version of libnvdimm.ko or nfit.ko due to
symbols not being mocked per nfit_test's expectation. For example,
nfit_test expects acpi_evaluate_dsm() to be replaced by
__wrap_acpi_evaluate_dsm() to test how acpi_nfit_ctl() responds to
different stimuli.

Create a test-only symbol name that nfit_test links against to cause
module load failures when the wrong module is present.

For example, with this change, attempts to use the wrong module will
report:

    nfit_test: Unknown symbol libnvdimm_test (err 0)

Reported-by: Dave Jiang <dave.jiang@intel.com>
Reported-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

tools/testing/nvdimm/watermark.h

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright(c) 2018 Intel Corporation. All rights reserved.
+#ifndef _TEST_NVDIMM_WATERMARK_H_
+#define _TEST_NVDIMM_WATERMARK_H_
+int pmem_test(void);
+int libnvdimm_test(void);
+int acpi_nfit_test(void);
+int device_dax_test(void);
+
+/*
+ * dummy routine for nfit_test to validate it is linking to the properly
+ * mocked module and not the standard one from the base tree.
+ */
+#define nfit_test_watermark(x)				\
+int x##_test(void)					\
+{							\
+	pr_debug("%s for nfit_test\n", KBUILD_MODNAME);	\
+	return 0;					\
+}							\
+EXPORT_SYMBOL(x##_test)
+#endif /* _TEST_NVDIMM_WATERMARK_H_ */