tests: Split out those tests requiring float and import.

Tests in basics (which should probably be renamed to core) should not
rely on float, or import any non-built-in files.  This way these tests
can be run when those features are not available.

All test in basics now pass on the pyboard using stmhal port, except for
string-repr which has some issues with character hex printing.

tests/float/int-divzero.py

@@ -0,0 +1,4 @@
+try:
+    1 / 0
+except ZeroDivisionError:
+    print("ZeroDivisionError")