Rework custom test machinery to give the individual tests more control

on how they're run, and combine environment.json and check.py

test/shared/utils.py

@@ -0,0 +1,25 @@
+import subprocess, sys
+
+def cibuildwheel_get_build_identifiers(project_path, env=None):
+    '''
+    Returns the list of build identifiers that cibuildwheel will try to build
+    for the current platform.
+    '''
+    cmd_output = subprocess.check_output(
+        [sys.executable, '-m', 'cibuildwheel', '--print-build-identifiers', project_path],
+        universal_newlines=True,
+        env=env,
+    )
+
+    return cmd_output.strip().split('\n')
+
+platform = None
+
+if sys.platform.startswith('linux'):
+    platform = 'linux'
+elif sys.platform.startswith('darwin'):
+    platform = 'macos'
+elif sys.platform in ['win32', 'cygwin']:
+    platform = 'windows'
+else:
+    raise Exception('Unsupported platform')