cmTestGenerator: Separate test properties for each configuration

Move property generation from GenerateScriptConfigs to separate copies
in GenerateOldStyle and GenerateScriptForConfig.  This causes the
per-config tests generated for the add_test(NAME) signature to each get
their own test properties.  This will allow us to later change the
property values based on the test configuration.

While at it, generate lower-case CMake code (set_tests_properties).

Inspired-by: Ben Boeckel <mathstuf@gmail.com>
cmTestGenerator: Evaluate generator expressions in test properties

This is useful for something like:

    set_tests_properties(mytest
      PROPERTIES
        WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/$<CONFIGURATION>")

The common thought is that getting CTEST_CONFIGURATION_TYPE to be
expanded when setting the property is the way, but this variable is not
normalized. On Windows, using the variable in `ctest -R rELEASE` will
likely work, but with Xcode, the path does not exist due to case
sensitivity.
add_test: Mention generator expressions in old-style add_test docs

The old-style add_test() call does not support generator expressions at
all. This also applies to the properties for the test, but it is not
mentioned at all.
Merge branch 'test-property-genex' into generate-modern-style

Resolve conflict in Source/cmTestGenerator.cxx by taking "their" side
(test-property-genex).  It already accounts for the lower-case change in
"our" side (generate-modern-style).
Merge topic 'generate-modern-style' into next

7a5a0ab Merge branch 'test-property-genex' into generate-modern-style
2dd34f6 add_test: Mention generator expressions in old-style add_test docs
0793463 cmTestGenerator: Evaluate generator expressions in test properties
dad426c cmTestGenerator: Separate test properties for each configuration