Tests: Fix standalone build of tests with nested projects

Since commit fd6076d0 (Tests: Pass CMAKE_MAKE_PROGRAM instead of
--build-makeprogram, 2013-11-15) the ExportImport, Fortran, and
MacRuntimePath tests use the value of CMAKE_TEST_MAKEPROGRAM as the
CMAKE_MAKE_PROGRAM for their nested projects configurations.
Teach these tests to initialize CMAKE_TEST_MAKEPROGRAM when it is
not provided, such as when building the tests manually.
Merge topic 'cleanup-build-commands'

ef7c11e Tests: Fix standalone build of tests with nested projects