Add a new test for flang

Build and run 2 simple Fortran programs using flang in order to
guarantee we have a flang that does the most basic tasks.

tests/flang/hello.f

@@ -0,0 +1,3 @@
+      PROGRAM HELLO
+        PRINT '(A)', 'Hello, world'
+      END

tests/flang/main.fmf

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+summary: Run simple tests for flang
+
+tag+:
+  - fedora-only
+
+adjust+:
+  - because: "flang is available for Fedora, not RHEL/CentOS"
+    when: >-
+      distro == rhel
+      or distro == centos
+    enabled: false
+
+  - because: "flang is not built for compat packages"
+    enabled: false
+    when: compat is defined
+
+require+:
+  - flang
+  - llvm-devel
+duration: 1h

tests/flang/sin.f90

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+program sin
+    real :: pi
+    pi = 4*atan(1.0)
+    print *, sin(pi/2)
+end program

tests/flang/test.sh

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+#!/usr/bin/bash
+# Compile and run simple Fortran programs in order to evaluate if Flang is
+# minimally funcional.
+
+set -e
+
+flang hello.f -o hello
+./hello | grep -q "Hello, world"
+rm hello
+
+flang sin.f90 -o sin
+./sin | grep -q "1."
+rm sin