GoogleTest: Optimize gtest_discover_tests

Prior to this, `gtest_discover_tests` could take multiple minutes if
many tests are present.  This behavior was caused by a repeated addition
to the variable `script` in the `add_command` function using:

    set(script "${script}${NAME}(${_args})\n" PARENT_SCOPE)

This takes very long for large variables.

This commit flushes the contents of the variable to ${CTEST_FILE} after
a certain size of the variable is reached.

In addition:

- cmake_minimum_required(VERSION ${CMAKE_VERSION}) is set to allow usage
  of new policies.  In particular, CMP0053 speeds up variable expansion.
- No longer appends strings using set(), but instead uses string(APPEND).
- An additional buffer for the tests variable is set.

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