Split tool test generators into core/header files

[mattjala]

This will allow gentest scripts to share routines between each other, so that cases where multiple tools tests require the same file can be handled without directly copying the files or duplicating file generation routines.

• Split the tool gentest files into core, executable, and header portions.

The core file for each tool contains the definitions for the routines that generate test files.

The executable file is built into the gentest script, which uses the routines defined in each core file.

The header declares the routines and any values that need to be shared between the executable and the core file.

[mattjala]

These entries were removed from the FILENAME_ENHANCE array since they're used in a different manner, and because doing so allowed the core/script split to be done more cleanly. Entries 6 and 7 were only used in main, while 0-5 were only used in the routines.

[mattjala]

• Moved the 'core' definition files back to the original h5gentest.c files
• Merged the actual scripts into h5gentest.c, which takes an argument to decide which sets of files to create

h5clear_gentest was excluded from h5gentest because it exits in a specific way to leave IDs open.
Similarly, h5perf_gentest was excluded because it doesn't seem to be used for h5perf testing in the same way as the other gentest scripts.

[jhendersonHDF]

Just a couple minor comments but this looks pretty good to me.