MDEV-20787: Script dgcov.pl does not workbb-10.2-MDEV-20787

- This patch is done after doing test on 00f9df29224ada9cdbb796c99f4
where difference was found between versions of gcov.
- This patch doesn't take gcov versions in account since is extracting
filename of a `.gcov` file, which is generated with `gcov -i
<source-file>.gcda` during `dgcov.pl --generate`.
- There is a check if file exists.
Reason for that is because some generated `gcov` files are outliers from above
rule (that a file generated in intermediate format(gcov) has the same source name).
Example 1: `aestables.cpp.gcda` is not generating `aestables.cpp.gcov`,
but only the files used in #include `modes.hpp.gcov  runtime.hpp.gcov`
Example 2: `my_sha256.cc.gcda` is generating `my_sha.ic.gcov`
- Make sure to test `MTR_BINDIR` which gets set during out-of-source
build and generate error when running `./mysql-test/mtr --gcov alias`
- During the testing:
  10.1 results are stored in
  - `<basedir>//mysql-test-gcov.msg` and errors in
  - `<basedir>//mysql-test-gcov.err`
  10.2 results should be stored in
  `./mysql-test/var/last_changes.dgcov`, but only empty results are
  obtained (tests `1st` and `alias`).

1 files changed, 1 insertions, 1 deletions

diff --git a/mysql-test/mysql-test-run.pl b/mysql-test/mysql-test-run.pl
index 178d65aca41..be51a79e0be 100755
--- a/mysql-test/mysql-test-run.pl
+++ b/mysql-test/mysql-test-run.pl
@@ -1791,7 +1791,7 @@ sub command_line_setup {
   # --------------------------------------------------------------------------
   # Gcov flag
   # --------------------------------------------------------------------------
-  if ( ($opt_gcov or $opt_gprof) and ! $source_dist )
+  if ( ($opt_gcov or $opt_gprof) and (! $source_dist or -d $ENV{MTR_BINDIR}))
   {
     mtr_error("Coverage test needs the source - please use source dist");
   }