Added a unit test for BcbCounters

Restructured the code a little, to allow getting both the mir::Body and
coverage graph.

compiler/rustc_mir/src/transform/coverage/counters.rs

@@ -120,7 +120,6 @@ struct BcbCounters<'a> {
     basic_coverage_blocks: &'a mut CoverageGraph,
 }
 
-// FIXME(richkadel): Add unit tests for `BcbCounters` functions/algorithms.
 impl<'a> BcbCounters<'a> {
     fn new(
         coverage_counters: &'a mut CoverageCounters,

compiler/rustc_mir/src/transform/coverage/spans.rs

@@ -645,7 +645,10 @@ impl<'a, 'tcx> CoverageSpans<'a, 'tcx> {
     }
 }
 
-fn filtered_statement_span(statement: &'a Statement<'tcx>, body_span: Span) -> Option<Span> {
+pub(super) fn filtered_statement_span(
+    statement: &'a Statement<'tcx>,
+    body_span: Span,
+) -> Option<Span> {
     match statement.kind {
         // These statements have spans that are often outside the scope of the executed source code
         // for their parent `BasicBlock`.
@@ -686,7 +689,10 @@ fn filtered_statement_span(statement: &'a Statement<'tcx>, body_span: Span) -> O
     }
 }
 
-fn filtered_terminator_span(terminator: &'a Terminator<'tcx>, body_span: Span) -> Option<Span> {
+pub(super) fn filtered_terminator_span(
+    terminator: &'a Terminator<'tcx>,
+    body_span: Span,
+) -> Option<Span> {
     match terminator.kind {
         // These terminators have spans that don't positively contribute to computing a reasonable
         // span of actually executed source code. (For example, SwitchInt terminators extracted from