Refactor the skipping conditions in the tests.

tests/pio_cksum_writer.test.in

 #!@SHELL@
-test 'x@ENABLE_GRIB@' = xyes || exit 77
 
 pio_write_args="-p PIO_WRITER -w 4"
 mpi_task_num=8

tests/test_cdf_const.in

 #!@SHELL@
-test 'x@ENABLE_NETCDF@' = xyes || exit 77
+@ENABLE_NETCDF_FALSE@exit 77
 
 cdi_app_stdout="$0.cdi_app.stdout"
 cdi_app_stderr="$0.cdi_app.stderr"

tests/test_cdf_transformation.in

 #!@SHELL@
-test 'x@ENABLE_NETCDF@' = xyes || exit 77
+@ENABLE_NETCDF_FALSE@exit 77
 
 ncdatafile="$0.nc"
 

tests/test_cksum_nc.in

 #!@SHELL@
-test 'x@ENABLE_NETCDF@' = xyes || exit 77
+@ENABLE_NETCDF_FALSE@exit 77
 
 prefix=${0##*/}
 format=nc

tests/test_cksum_nc_chunk.in

 #!@SHELL@
-test 'x@ENABLE_NETCDF@' = xyes || exit 77
+@ENABLE_NETCDF_FALSE@exit 77
 
 prefix=${0##*/}
 format=nc

tests/test_f2003.in

 #!@SHELL@
 @ENABLE_ISOC_INTERFACE_FALSE@exit 77
-test 'x@ENABLE_NETCDF@' = xyes || exit 77
+@ENABLE_NETCDF_FALSE@exit 77
 
 cdi_read_f2003_stdout="$0.cdi_read_f2003.stdout"
 cdi_read_f2003_stderr="$0.cdi_read_f2003.stderr"