These tests were not being run when -D_GLIBCXX_USE_CXX11_ABI=0 was added
to the test flags, but they actually work OK with the old string.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/basic_string/allocator/char/minimal.cc:
	Do not require cxx11-abi effective target.
	* testsuite/21_strings/basic_string/allocator/wchar_t/minimal.cc:
	Likewise.
	* testsuite/27_io/basic_fstream/cons/base.cc: Likewise.

Tested powerpc64le-linux. Committed to trunk.