[test] fix rounding of benchmarks seconds

some OS have no hires timings on benchmarks.
thus not all floats are printed with a ., esp. on cygwin.
GH #1066.

see http://perl514.cpanel.net/build/builders/parrot-cygwin/builds/41/steps/shell_2/logs/stdio
"0 seconds. 4000 lops/sec"