Move libm-test TEST_MSG definitions to libm-test-driver.c.

Various files using the libm-test infrastructure define a TEST_MSG
macro with an informal description of the tests being run.

This patch moves this macro to libm-test-driver.c (the definition
depending on other macros already defined), so files specific to
(type, choice of whether to test inline functions or finite-math-only
functions, vector length) no longer need to define it.  This is in
preparation for replacing files such as test-float.c with per-function
test-float-<func>.c etc. automatically generated in the build
directory when tests are run.

Tested for x86_64.

	* math/libm-test-driver.c (STRX): New macro.
	(STR): Likewise.
	(STR_FLOAT): Likewise.
	(STR_VEC_LEN): Likewise.
	(TEST_MSG): Likewise.  Define here instead of expecting to be
	defined by including file.
	* math/test-double-finite.c (TEST_MSG): Remove macro.
	* math/test-double-vlen2.h (TEST_MSG): Likewise.
	* math/test-double-vlen4.h (TEST_MSG): Likewise.
	* math/test-double-vlen8.h (TEST_MSG): Likewise.
	* math/test-double.c (TEST_MSG): Likewise.
	* math/test-float-finite.c (TEST_MSG): Likewise.
	* math/test-float-vlen16.h (TEST_MSG): Likewise.
	* math/test-float-vlen4.h (TEST_MSG): Likewise.
	* math/test-float-vlen8.h (TEST_MSG): Likewise.
	* math/test-float.c (TEST_MSG): Likewise.
	* math/test-idouble.c (TEST_MSG): Likewise.
	* math/test-ifloat.c (TEST_MSG): Likewise.
	* math/test-ildouble.c (TEST_MSG): Likewise.
	* math/test-ldouble-finite.c (TEST_MSG): Likewise.
	* math/test-ldouble.c (TEST_MSG): Likewise.