With Maciej's change, $tests_time is one large integer instead of a float
with the pattern sssssssssssnnnnnnnnn (ie., currently, 10 digits for the
Unix time seconds and always 9 digits for the nanoseconds.)

${#tests_time} returns the length of $tests_time (ie., currently, 19.)
 Substring expansion in Bash supports arithmetic expressions, so
${#tests_time}-9 is properly calculated (to 10) and thus cuts the seconds
from the nanoseconds.