RFR: Fixed issue when jasm produced only one file from source with multiple ones

[Jiří Vaněk]

Added tests for this issue
Added test veryfying that the jcoder is not affected

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Commit messages:
 - Fixed issue when jasm produced only one file from source with multiple ones

Changes: https://git.openjdk.org/asmtools/pull/49/files
 Webrev: https://webrevs.openjdk.org/?repo=asmtools&pr=49&range=00
  Stats: 1049 lines in 8 files changed: 1044 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/asmtools/pull/49.diff
  Fetch: git fetch https://git.openjdk.org/asmtools pull/49/head:pull/49

PR: https://git.openjdk.org/asmtools/pull/49