String Equality tests in tests/main.cpp

[zbud-msft]

Currently in tests/main.cpp, we are testing corner cases with a few SonicDBConfig functions, and we test that the string from the exception that is caught, matches an expected string.

https://github.com/sonic-net/sonic-swss-common/blob/master/tests/main.cpp#L71

However, this is not robust and better practice would be to test for what exception type is thrown as exception strings can be easily changed breaking the test or having to change in multiple locations.

https://github.com/sonic-net/sonic-swss-common/blob/master/common/dbconnector.cpp#L248

Here we see out_of_range exception thrown; we can use EXPECT_THROW instead to test that the same exception is thrown in our UT.