Fix usage of find in tests/Makefile.am

The path is not optional on FreeBSD.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes openzfs#12453

tests/Makefile.am

@@ -4,5 +4,5 @@ SUBDIRS = runfiles test-runner zfs-tests
 
 EXTRA_DIST = README.md
 
-SHELLCHECKSCRIPTS = $$(find -name '*.sh')
+SHELLCHECKSCRIPTS = $$(find . -name '*.sh')
 .PHONY: $(SHELLCHECKSCRIPTS)