Rename test modules to "tests" for consistency.

The majority of the test modules were already named "tests", and that's
what the example in the Rust book uses, so switch to that for all test
modules, for consistency.

lib/codegen/src/dominator_tree.rs

@@ -666,7 +666,7 @@ impl DominatorTreePreorder {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::*;
     use cursor::{Cursor, FuncCursor};
     use flowgraph::ControlFlowGraph;

lib/codegen/src/ir/libcall.rs

@@ -190,7 +190,7 @@ fn make_funcref(libcall: LibCall, func: &mut Function, sig: Signature, isa: &Tar
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::*;
     use std::string::ToString;
 

lib/codegen/src/loop_analysis.rs

@@ -230,8 +230,7 @@ impl LoopAnalysis {
 }
 
 #[cfg(test)]
-mod test {
-
+mod tests {
     use cursor::{Cursor, FuncCursor};
     use dominator_tree::DominatorTree;
     use flowgraph::ControlFlowGraph;

lib/codegen/src/regalloc/virtregs.rs

@@ -396,7 +396,7 @@ impl VirtRegs {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::*;
     use entity::EntityRef;
     use ir::Value;

lib/codegen/src/timing.rs

@@ -242,7 +242,7 @@ mod details {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::*;
     use std::string::ToString;
 

lib/codegen/src/topo_order.rs

@@ -88,7 +88,7 @@ impl TopoOrder {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::*;
     use cursor::{Cursor, FuncCursor};
     use dominator_tree::DominatorTree;