Use Ninja-compatible syntax for passing TESTOPTS

Ninja doesn't support passing environment as command line arguments
like make does, access TESTOPTS through environment instead of the
make syntax to make it work for both generators.

tests/CMakeLists.txt

@@ -81,7 +81,7 @@ if (MKTREE_BACKEND STREQUAL oci)
 		configure_file(${DOCKERFILE} Dockerfile COPYONLY)
 		add_custom_target(ci
 			COMMAND ./mktree.oci build
-			COMMAND ./mktree.oci check ${JOBS} $(TESTOPTS)
+			COMMAND ./mktree.oci check ${JOBS} $ENV{TESTOPTS}
 			WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
 		)
 	else()
@@ -143,7 +143,7 @@ add_custom_target(shell
 )
 
 add_custom_target(check
-	COMMAND ./mktree check ${JOBS} $(TESTOPTS)
+	COMMAND ./mktree check ${JOBS} $ENV{TESTOPTS}
 	DEPENDS tree
 )
 

tests/README.md

@@ -59,6 +59,11 @@ For all available options, see the output of the command:
 By default, tests are executed in parallel using all available cores, pass a
 specific `-jN` value to limit.
 
+Note that when building with Ninja, TESTOPTS must be passed via the
+environment (also works with make):
+
+    TESTOPTS="..." ninja check
+
 ## Interactive testing
 
 To drop into an interactive GNU Autotest shell, run: