fix: the way run_tests.sh works with arguments

Closes #50 (closed)

Examples of wrong usage:

./run_tests.sh -t
Option -t requires an argument.

./run_tests.sh -t tests/default.tftest.hcl -t
Option -t requires an argument.

./run_tests.sh -v -t
Option -t requires an argument.

./run_tests.sh -G
Invalid option: -G

Example of correct usage

will run all cases (default with no flags)

./run_tests.sh
[+] Building 0.0s (0/0) ... so on ...

ci mode

./run_tests.sh -c -v
CI mode: true
Verbose: true
... so on ...

verbose mode, 2 tests specified

./run_tests.sh -v -t tests/cloudsql.tftest.hcl -t tests/cloud_run_service.tftest.hcl
CI mode: false
Verbose: true
[+] Building 0.0s (0/0)                                                                                                                                          
[+] Building 0.0s (0/0) 
... so on ...