libstdc++: Fix tests that fail with fully-dynamic-string

Fix some tests that assume that a moved-from string is empty, or that
default constructing a string doesn't allocate.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/basic_string/cons/char/moveable.cc: Allow
	moved-from string to be non-empty.
	* testsuite/21_strings/basic_string/cons/char/moveable2.cc:
	Likewise.
	* testsuite/21_strings/basic_string/cons/char/moveable2_c++17.cc:
	Likewise.
	* testsuite/21_strings/basic_string/cons/wchar_t/moveable.cc:
	Likewise.
	* testsuite/21_strings/basic_string/cons/wchar_t/moveable2.cc:
	Likewise.
	* testsuite/21_strings/basic_string/cons/wchar_t/moveable2_c++17.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/assign/char/87749.cc:
	Construct empty string before setting oom flag.
	* testsuite/21_strings/basic_string/modifiers/assign/wchar_t/87749.cc:
	Likewise.