Update integration test script

[joehu21]

Enhance the integration test script to check if environment variable SERVER_VERSION is set. If not, default to "unstable". Also update the README for how to run one integration test.

Details:

1. Running the integration test using "make test" failed:

Running integration tests against Valkey version: 
.build/binaries//valkey-server missing
make[3]: *** [CMakeFiles/test] Error 1
make[2]: *** [CMakeFiles/test.dir/all] Error 2
make[1]: *** [CMakeFiles/test.dir/rule] Error 2
make: *** [test] Error 2

2. The root cause is that environment variable SERVER_VERSION is missing.
3. Fixed by checking if environment variable SERVER_VERSION is set. If not, default to "unstable".

[roshkhatri]

Looks good to me!