CTest: Remove need of usage of two variables in Coverage

Before, there were two variables for CoverageCommand and
CoverageExtraFlags. Both of them were needed to be used if
both script file and console is used for ctest.

COVERAGE_COMMAND & CTEST_COVERAGE_COMMAND
COVERAGE_EXTRA_FLAGS & CTEST_COVERAGE_EXTRA_FLAGS

In DartConfiguration file, the former ones were used to replace
the values. However, if you use a script file to do coverage on
source, script looks for the latter ones which fails the coverage
as they were not set (except for using set(COVERAGE_COMMAND "${CTEST_COVERAGE_COMMAND}")
in script).

Now, CTEST_COVERAGE_COMMAND has precedence over COVERAGE_COMMAND which
assigns itself to COVERAGE_COMMAND if COVERAGE_COMMAND is not set
explicitly. For backward compatibility, if CTEST_COVERAGE_COMMAND is
also not set, COVERAGE_COMMAND is reset to gcov and the FLAGS are
reset to -l.