tst: Extend cross-test-ssh.sh to specify if target date can be altered

tst: Extend cross-test-ssh.sh to specify if target date can be altered

This code adds new flag - '--allow-time-setting' to cross-test-ssh.sh
script to indicate if it is allowed to alter the date on the system
on which tests are executed. This change is supposed to be used with
test systems, which use virtual machines for testing.

The GLIBC_TEST_ALLOW_TIME_SETTING env variable is exported to the
remote environment on which the eligible test is run and brings no
functional change when it is not.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>