[flang][NFC] Allow whitespaces before `ERROR`

[unterumarmung]

Summary

This change allows to write whitespaces before the ERROR keyword
in semantic tests for consistency with other testing infrastructure.

Also, one test is changed in order to test if the change works
correctly.

[Meinersbur]

LGTM

flang/test/Semantics/altreturn04.f90

 ! RUN: %python %S/test_errors.py %s %flang_fc1
 ! Functions cannot use alt return
 
 REAL FUNCTION altreturn01(X)
-!ERROR: RETURN with expression is only allowed in SUBROUTINE subprogram
+! ERROR: RETURN with expression is only allowed in SUBROUTINE subprogram
   RETURN 1
 END

flang/test/Semantics/test_errors.py

 for line in log.split('\n'):
     m = re.search(r"[^:]*:(\d+:).*(?:error:)(.*)", line)
     if m:
         actual += m.expand(r"\1\2\n")
 
 # Gets the expected errors and their line number
 errors = []
 for i, line in enumerate(src, 1):
-    m = re.search(r"(?:^\s*!ERROR: )(.*)", line)
+    m = re.search(r"(?:^\s*!\s*ERROR: )(.*)", line)
     if m:
         errors.append(m.group(1))
         continue
     if errors:
         for x in errors:
             expect += f"{i}: {x}\n"
         errors = []