glslang: add run_tests.sh

[DavidKorczynski]

run_tests.sh is used as part of Chronos with cached builds: https://github.com/google/oss-fuzz/tree/master/infra/experimental/chronos#check-tests

$ ./infra/experimental/chronos/check_tests.sh glslang c++
...
...
+ cd build                                                                                                                                                                                                                                                         
+ make test                                                                                                                                                                                                                                                        
Running tests...                                                                                                                                                                                                                                                   
/usr/local/bin/ctest --force-new-ctest-process                                                                                                                                                                                                                     
Test project /src/glslang/build                                                                                                                                                                                                                                    
    Start 1: glslang-testsuite                                                                                                                                                                                                                                     
1/1 Test #1: glslang-testsuite ................   Passed   35.85 sec                                                             
                                                                                                                                 
100% tests passed, 0 tests failed out of 1                                                                                                                                                                                                                         
                                                                                                                                                                                                                                                                   
Total Test time (real) =  35.85 sec