tests: remove deprecated import('unstable_')

test cases/common/230 external project/libfoo/meson.build

@@ -1,4 +1,4 @@
-mod = import('unstable_external_project')
+mod = import('unstable-external_project')
 
 target_system = target_machine.system()
 if target_system in ['windows', 'cygwin']

test cases/fpga/1 simple/meson.build

@@ -1,6 +1,6 @@
 project('lattice', 'c')
 
-is = import('unstable_icestorm')
+is = import('unstable-icestorm')
 
 is.project('spin',
   'spin.v',