test: skip instead of fail when mem constrained

The current implementation of tests for strings with length at or
exceeding kStringMaxLength allocate a temporary buffer inside a try
block to skip the test if there is insufficient memory. This commit adds
an invocation of the garbage collector after the temporary buffer is
allocated so that memory is freed for later allocations.

Change the corresponding catch block to rethrow the original exception
instead of asserting the exception message to provide more information
about the exception.

Add an additional check before trying to allocate memory to immediately
skip the test on machines with insufficient total memory.

PR-URL: https://github.com/nodejs/node/pull/3697


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>