Makefile: make tests posix compliant

Summary:
The -E option to find doesn't exist on linux, so I rewrote the
regular expression used for finding test files as a basic regular
expression that should work with any posix-compliant find.

Hopefully this makes `make test` and `make subperseus` work on
linux.

Test Plan:
`make test` see that it works
hope it works on linux too

Auditors: michelle