The function gtest_discover_tests calls the passed test executable with
the parameter --gtest_list_tests and parses the output to find all
tests.

In case of value-parameterized tests ([1]), the test values are included
in the output. While test names are alphanumeric, the values can contain
arbitrary content.

First, the output is separated into lines with `foreach`. Included
semi-colons breaks this and need to get escaped.

Afterwards, the testname is passed on to the `add_command` helper. This
helper was converted into a macro in commit dac20144 (GoogleTest:
Optimize gtest_discover_tests, 2020-02-18). As a macro, its arguments
are re-evaluated. Therefore we need to escape `\`, `;` and to prevent
unwanted variable expansion `$`.

Fixes: #20661

[1] <https://github.com/google/googletest/blob/0eea2e9/googletest/docs/advanced.md#value-parameterized-tests>