Move env var tests into separate modules

Tests that modify environment variables (e.g. CFLAGS and CXXFLAGS) can
cause tests to fail when run in parallel because they change a shared
context.

This change moves the problematic tests into separate modules. Since
each `tests/*.rs` is compiled as a separate crate, these tests are not
run in parallel with other tests.

tests/cflags.rs

@@ -0,0 +1,18 @@
+extern crate cc;
+extern crate tempdir;
+
+mod support;
+
+use std::env;
+use support::Test;
+
+/// This test is in its own module because it modifies the environment and would affect other tests
+/// when run in parallel with them.
+#[test]
+fn gnu_no_warnings_if_cflags() {
+    env::set_var("CFLAGS", "-arbitrary");
+    let test = Test::gnu();
+    test.gcc().file("foo.c").compile("foo");
+
+    test.cmd(0).must_not_have("-Wall").must_not_have("-Wextra");
+}

tests/cxxflags.rs

@@ -0,0 +1,18 @@
+extern crate cc;
+extern crate tempdir;
+
+mod support;
+
+use std::env;
+use support::Test;
+
+/// This test is in its own module because it modifies the environment and would affect other tests
+/// when run in parallel with them.
+#[test]
+fn gnu_no_warnings_if_cxxflags() {
+    env::set_var("CXXFLAGS", "-arbitrary");
+    let test = Test::gnu();
+    test.gcc().file("foo.cpp").cpp(true).compile("foo");
+
+    test.cmd(0).must_not_have("-Wall").must_not_have("-Wextra");
+}

tests/test.rs

@@ -1,7 +1,6 @@
 extern crate cc;
 extern crate tempdir;
 
-use std::env;
 use support::Test;
 
 mod support;
@@ -111,26 +110,6 @@ fn gnu_warnings_overridable() {
         .must_have_in_order("-Wall", "-Wno-missing-field-initializers");
 }
 
-#[test]
-fn gnu_no_warnings_if_cflags() {
-    env::set_var("CFLAGS", "-Wflag-does-not-exist");
-    let test = Test::gnu();
-    test.gcc().file("foo.c").compile("foo");
-
-    test.cmd(0).must_not_have("-Wall").must_not_have("-Wextra");
-    env::set_var("CFLAGS", "");
-}
-
-#[test]
-fn gnu_no_warnings_if_cxxflags() {
-    env::set_var("CXXFLAGS", "-Wflag-does-not-exist");
-    let test = Test::gnu();
-    test.gcc().file("foo.c").compile("foo");
-
-    test.cmd(0).must_not_have("-Wall").must_not_have("-Wextra");
-    env::set_var("CXXFLAGS", "");
-}
-
 #[test]
 fn gnu_x86_64() {
     for vendor in &["unknown-linux-gnu", "apple-darwin"] {