[flang][NFC] Allow whitespaces before ERROR

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.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D125884

Author: Daniil Dudkin

flang/test/Semantics/altreturn04.f90

@@ -2,6 +2,6 @@
 ! 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

@@ -49,7 +49,7 @@
 # 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