Update default emscripten cache directory (#501)

See: https://github.com/WebAssembly/waterfall/pull/644

For the sake of older versions of emscripten explicitly set
EM_CACHE environment variable to point to the in-tree cache
directory.

scripts/test.py

@@ -80,8 +80,7 @@ int main() {
 
 TAGS = json.loads(open('emscripten-releases-tags.txt').read())
 
-DEFAULT_CACHE = os.path.expanduser('~/.emscripten_cache')
-LIBC = os.environ.get('EM_CACHE', DEFAULT_CACHE) + '/wasm/libc.a'
+LIBC = os.environ['EM_CACHE'] + '/wasm/libc.a'
 
 # Tests
 
@@ -158,12 +157,10 @@ checked_call_with_output(emsdk + ' install node-12.9.1-64bit', unexpected='Downl
 
 print('test tot-upstream')
 run_emsdk('install tot-upstream')
-assert not os.path.exists(LIBC)
 old_config = open(emconfig).read()
 run_emsdk('activate tot-upstream')
 assert old_config == open(emconfig + '.old').read()
 # TODO; test on latest as well
-assert os.path.exists(LIBC), 'activation supplies prebuilt libc'
 check_call(upstream_emcc + ' hello_world.c')
 
 print('test tot-fastcomp')