Modernize `cmCTest.cxx`

• make cmCTest::Log() accepts string, so logging macros cmCTestLog and cmCTestOptionalLog do not need to call c_str() on a string obtained from std::stringstream;
• remove useless "Here: << __LINE__ debug messages;
• cmsys::?fstream accept strings, so no need to call c_str();
• avoid macros in cmCTest::Log replaced w/ lambda;
• simplify logType selection logic in cmCTest::Log;
• some other little simplifications;
• add tests for cmCTest::decodeURL.

Topic-rename: ctest-cleanup