Start child processes one at a time; run gcov without a directory

Reading the coverage profiles on several threads broke an assumption of
ChildProcess::Start on POSIX: it changes the working directory of the
whole process around posix_spawn. Two gcov runs started at once could
both run in one test's directory, so on the GCC job the map of
"Subtracts one" showed what "Adds one" reached.

- ChildProcess::Start holds a lock while a child starts, on POSIX and
  Windows, so the directory change and the handles a child inherits never
  overlap with another start.
- gcov gets the absolute path of the data file and no working directory,
  so no directory changes while profiles are read in parallel.

1 files changed, 10 insertions, 8 deletions

diff --git a/src/Progmasoft/Runner/CoverageReport.cpp b/src/Progmasoft/Runner/CoverageReport.cpp
index fb2760c3..ccdd0538 100644
--- a/src/Progmasoft/Runner/CoverageReport.cpp
+++ b/src/Progmasoft/Runner/CoverageReport.cpp
@@ -239,14 +239,16 @@ namespace Progmasoft::Catch3::Runner
                     return Error{ "the notes file " + gcno.string()
                                   + " could not be read: " + error.message() };
                 }
-                auto json = RunTool(
-                    gcov,
-                    // Without --branch-probabilities the JSON has no branches.
-                    { "--json-format",
-                      "--branch-probabilities",
-                      "--stdout",
-                      gcda.filename().string() },
-                    gcda.parent_path());
+                // The data file by its absolute path, so that gcov needs no
+                // working directory: profiles are read on several threads,
+                // and a working directory is the whole process's.
+                // Without --branch-probabilities the JSON has no branches.
+                auto json
+                    = RunTool(gcov,
+                              { "--json-format",
+                                "--branch-probabilities",
+                                "--stdout",
+                                std::filesystem::absolute(gcda).string() });
                 if (!json)
                 {
                     return json.GetError();