tests: check SonicDBConfig exception types

[dlanov]

What

Replace exception-message string comparisons in tests/main.cpp with EXPECT_THROW assertions using the expected exception types.

Fixes #857.

Why

The existing tests depend on exact exception text, which makes them fragile when error messages change. The manual try/catch blocks also do not fail if the expected call stops throwing altogether.

How

• Check missing database config with std::runtime_error.
• Check access before global database config initialization with std::runtime_error.
• Check an invalid namespace/key with std::out_of_range.
• Remove the obsolete exception-message comparisons.

Testing

Built the unit-test binary with:

./autogen.sh
./configure --enable-debug --disable-yangmodules 'CXXFLAGS=-O0 -g'
make common/cfg_schema.h
GCC_COLORS=1 make -j16 tests/tests

Ran:

./tests/tests --gtest_filter='STRINGUTILITY.*'

Result:

[PASSED] 7 tests.

The global SwsscommonEnvironment::SetUp() executed all three updated exception checks successfully.

[mssonicbld]

/azp run

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[mssonicbld]

Hi, there are workflow run(s) waiting for approval, you may be first-time contributor. I will notify maintainers to help approve once PR is approved. Thanks!

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