Description:
Added internal tesitng routines to traverse a file and verify that symbol table
information is *always* cached, whenever possible. Added this check to the end
of many tests to check all the test files, right before the call to h5_cleanup.
Tested: jam, amani, heiwa (h5committest)
1) Move the test for H5D_EXT_PREFIX in links.c to a separate file: links_env.c
2) links_env.c will be used by testlinks_env.sh to test for the environmental
variable H5D_EXT_PREFIX in searching for the external linked file.