Fix test_omp_cancellation_env_1.c

[tob2]

The OMP_CANCELLATION env variable has to be set to either true or false, other values like 1 are implemenation defined. Hence, the file name is changed from ...env_1.c to ..._env_true.c to make the test well-defined as that value is used to set env var.

Follow-up to Pull Request #812.

The OMP_CANCELLATION environment variable sets the initial value of the cancel-var ICV. The value of this environment variable must be one of the following: true|false. The behavior of the program is implementation defined if OMP_CANCELLATION is set to neither true nor false.

@fel-cab @spophale @seyonglee @andrewkallai – please have a look