tests/feature_check: Check for lack of pass result rather than failure.

Commit cb68a574 broke automatic Python
feature detection when running tests, because some detection relied on a
crash of a feature script returning exactly b"CRASH".

This commit fixes this and improves the situation by testing for the lack
of a known pass result, rather than an exact failure result.
Signed-off-by: Damien George <damien@micropython.org>

tests/feature_check/async_check.py

 # check if async/await keywords are supported
 async def foo():
     await 1
+
+
+print("async")

tests/feature_check/const.py

 x = const(1)
+print(x)

tests/feature_check/native_check.py

@@ -2,3 +2,7 @@
 @micropython.native
 def f():
     pass
+
+
+f()
+print("native")

tests/feature_check/set_check.py

 # check if set literal syntax is supported
-{1}
+print({1})

tests/run-tests.py

@@ -279,7 +279,7 @@ def run_tests(pyb, tests, args, result_dir):
 
         # Check if micropython.native is supported, and skip such tests if it's not
         output = run_feature_check(pyb, args, base_path, "native_check.py")
-        if output == b"CRASH":
+        if output != b"native\n":
             skip_native = True
 
         # Check if arbitrary-precision integers are supported, and skip such tests if it's not
@@ -294,7 +294,7 @@ def run_tests(pyb, tests, args, result_dir):
 
         # Check if set type (and set literals) is supported, and skip such tests if it's not
         output = run_feature_check(pyb, args, base_path, "set_check.py")
-        if output == b"CRASH":
+        if output != b"{1}\n":
             skip_set_type = True
 
         # Check if slice is supported, and skip such tests if it's not
@@ -304,12 +304,12 @@ def run_tests(pyb, tests, args, result_dir):
 
         # Check if async/await keywords are supported, and skip such tests if it's not
         output = run_feature_check(pyb, args, base_path, "async_check.py")
-        if output == b"CRASH":
+        if output != b"async\n":
             skip_async = True
 
         # Check if const keyword (MicroPython extension) is supported, and skip such tests if it's not
         output = run_feature_check(pyb, args, base_path, "const.py")
-        if output == b"CRASH":
+        if output != b"1\n":
             skip_const = True
 
         # Check if __rOP__ special methods are supported, and skip such tests if it's not
@@ -334,10 +334,10 @@ def run_tests(pyb, tests, args, result_dir):
 
         upy_byteorder = run_feature_check(pyb, args, base_path, "byteorder.py")
         upy_float_precision = run_feature_check(pyb, args, base_path, "float.py")
-        if upy_float_precision == b"CRASH":
-            upy_float_precision = 0
-        else:
+        try:
             upy_float_precision = int(upy_float_precision)
+        except ValueError:
+            upy_float_precision = 0
         has_complex = run_feature_check(pyb, args, base_path, "complex.py") == b"complex\n"
         has_coverage = run_feature_check(pyb, args, base_path, "coverage.py") == b"coverage\n"
         cpy_byteorder = subprocess.check_output(